RPi Expansion Boards
Hardware & Peripherals:
Hardware - detailed information about the Raspberry Pi boards.
Hardware History - guide to the Raspberry Pi models.
Low-level Peripherals - using the GPIO and other connectors.
Expansion Boards - GPIO plug-in boards providing additional functionality.
Screens - attaching a screen to the Raspberry Pi.
Cases - lots of nice cases to protect the Raspberry Pi.
Other Peripherals - all sorts of peripherals used with the Raspberry Pi.
On the Raspberry Pi, there are several connections which can be used for expansion:
- The Rpi GPIO (General Purpose Input/Output) pins are exposed, that means that expansion boards are able to talk directly to the CPU.
- The DSI connector will allow low-level interfacing with LCDs and other displays.
- The CSI connector will allow camera modules to be added in future.
Current expansion boards
Heber Ltd X10i is a universal, powerful and secure real-time controller that permits control over multiple inputs/outputs, via USB, from any PC system. Heber now offers support in both C/C++ to make it even easier for programmers, inventors and developers to connect with the outside world from their Raspberry Pi.
AFLEX Robotic Board
AFLEX Robotic Board is dual motor control and data acquisition board with I2C and Serial interfaces. The board provides 2 motor control drivers with max current up to 3.5A, 8-bits data port with each pin setup as digital input /output or analog input, 10-bits ADC with up to 5 analog channels, 4 inputs for the sensor connections, infrared (IR) remote control. The board provides direct connection to Raspberry Pi.
ADC Pi from AB Electronics UK brings 8 channel 15 bit analogue sampling to the Raspberry Pi platform. A buffered 5V i2c bus and stackable headers make this a useful addition to your Raspberry Pi project. http://www.abelectronics.co.uk/default.aspx
Arduino Shield and Raspberry Pi
a little board with Arduino socket and Raspberry Pi socket you can power the boards with 12Volt or 5Volt http://www.progetti-hw-sw.it/raspberry.htm
Buffer Board How to make a Pi Buffer on Veroboard. Each of the 14 outputs can drive up to 500mA using a Darlington array.
Buzzberry Buzz-Board Interface
Buzzberry is an interface board enabling expansion using the Buzz-Board modular system designed primarily for educational use.
On 2nd January 2012, CJE/4D announced that they have a real-time clock (RTC) expansion board in the works: 
- main page: RPi Gertboard
This board is a buffer between your Rpi and the real world, use it to flash LEDs on and off, drive motors, run sensors and all that other fun stuff.
GELI 'jelly'(GPIO Experimenter and Lab Interface Board)
Raspberry Geli Board Extends the GPIO and provides I2C, D2A/A2D, DC Motor(L298), RS232, Realtime Clock (DS1307), 1-wire and prototyping area on 150 * 100mmm PCB.
GPIO isolated expansion/RTC
UT Electronics is developing a expansion board containing a fully isolated 16-I/O I2C GPIO expander and RTC, with extensive options for further expansion. C and Python libraries will be available at launch. The board will probably be sold as a kit, with SMT components already in place.
Update: According to their website, the realities of life prevent them from bringing the product to market. They are hoping someone will pick up the ball. They have released the PCB layout and all documentation for free on their website.
“À la mode” is an Arduino clone specifically designed to interface with the Raspberry Pi. You can of course connect a standard Arduino to a Pi USB port using a cord, but when you want a turnkey solution, how about an Arduino compatible “plate” (shield) that fits right on top of the Raspberry Pi with direct access to GPIO port? The Linux side handles all of the displays, human interfaces, and number crunching. The low power Arduino compatible “à la mode” board handles sensors, servos/motors, and provides a highly accurate real time clock.
PiBorg is a Robotics add on board which adds the ability to control motors - from small to large (including DC motors, Stepper motors, BLDC motors and Solenoids) from your Raspberry Pi. It supports feedback via Encoders and Potentiometers, allowing closed loop (servo) control of motors to set positions, velocities and accelerations and it is capable of monitoring drive current and voltages. The loop is PID based and as PiBorg is an Open Source project, you may change the code as you please. It has the ability to drive 10A motors and more, so you'll never be left underpowered with this board!
Pi232 RS232 board
Pi232 is an RS232 expansion board that plugs onto GPIO connector.
Youtube video showing prototype board
Not yet tested, comments welcome Project files for Eagle here:  Schematic and Board images: [/File:PiTin_Schematic.png] [/File:PiTin_Boardpng.png] This board uses Microchip 16-pin GPIO expanders driven by SPI to provide up to 256 additional GPIO pins at a very low cost. Optionally, it can supply power for the board and the Raspberry PI.
The board can be made in a stackable manner allowing you to also place additional peripherals on the GPIO connector. It makes non-exclusive use of all four SPI pins as well as exclusive use of the CE0 Pin.
If you install (and power) the power supply components, the board will supply up to 1A at 5VDC. This is not enough to power all GPIO pins on a fully populated board, but in most applications should be sufficient to power the RPI and a reasonable number of GPIOs. An external power supply should be used for higher-power applications. This board should never be powered from the RPI as even one of the chips can draw more than the RPI can provide.
The power supply is quite flexible and will accept anything from 8-25VAC or 8-42VDC. It is based on a Recom module which provides a switching power supply in a form factor that is a drop-in replacement for 7805 series TO-220 linear regulators.
The 8 chips can be had for about $4@ or less and the power supply components total about $15. A fully populated board is about $49+PCB, $31+PCB for 64 GPIO pins.
You can have the PCB made at a variety of fabricators.  does very nice work and is hobbyist friendly at very reasonable prices.
Pi-Wire Homelabs are developing options for connection of a 1-wire bus to the RaspberryPi. Boards include SPI breakout connections for future boards.
PONTE initial post An arduino shield bridge from Omer and SKPang
Quick2Wire GPIO Expansion Board
Quick2Wire GPIO board and experimenters kit is an extension board for the Raspberry Pi that has level converters to allow the driving of TTL and real world devices such as LED's and switches. It will also provide an experimenters kit, a software framework and a number of tutorials.
Raspberry Pi IO Interface
forum thread showing 'paper mockup'. Another buffered IO expander.
Relay board and Raspberry Pi GPIO
a board with 8 relays and 8 input with a Raspberry Pi socket you can power the boards with 12Volt, the board has a DIN rail container http://www.progetti-hw-sw.it/raspberry.htm
 is a small (50x40mm) add-on board which provides a dual H-Bridge DC-Motor circuit using the venerable L293D chip.
 is a small (50x40mm) add-on board which provides a two 1A DPCO Relays. Each relay has a set of screw terminals for easy wiring.
MiniPiio DIO16 is a small (50x40mm) add-on board which provides an additional 16-channels of Digital Input/Outputs for the RPi. It can use either a MCP23S17 (SPI) or MCP23017 (I2C) Port Expander.
MiniPiio RS232 is a small (50x40mm) add-on board which provides a basic RS232 to TTL interface for the RPi. It uses a 3.3V MAX3232 to provide the RS232 interface.
MiniPiio ULN2803 is a small(50x40mm) add-on board which connects a ULN2803 Darlington Driver chip to the RPi's GPIO allowing the RPi to control electronic circuits which require more current to drive them.
RPi MIDI Breakout Board
RPi MIDI Breakout Board is a simple MIDI breakout board for the Raspberry Pi using the hardware UART from the RPi's expansion interface
RPi Protoboard SM
RPi UserPort is a digital and analogue I/O board for the Raspberry Pi. UserPort replicates the original BBC Micros digital User Port and Analogue Port but uses more modern components such as the MCP23S17 16-ch GPIO and MCP3204 4-ch 12-bit ADC.
BitWizard has a Raspberrypi serial breakout board. It breaks out four serial buses from the GPIO port.
BitWizard then has a series SPI (or I2C) expansion boards that can be daisy-chained off the SPI (or I2C) buses of the expansion board. A virtually unlimited number of expansion boards can be connected. Expansion boards are available that drive a simple 16x2 text LCD, more general purpose IOs, servos, temperature sensors, relays, optocouplers etc.
RPi RS232 Breakout Board
RPi RS232 Breakout Board is a simple RS232 breakout board for the Raspberry Pi
Slice of Pi
Slice of Pi Ciseco have produced a convienient small break out board for the Raspberry PI. XBee style connector for XRF / XBee / RN-XV / XBT / RF-BEE etc. Has the SPI and I2C pins on one standard 8 way header and the Raspberry PI's 8 general purpose I/O pins on another for easy access. Comes as a kit of parts that needs to be soldered. Construction is very easy and suitable for children with basic soldering skills.
XBee ZB SMT Backpack
The Raspberry Pi XBee ZB SMT Backpack by bootc is a carrier board for Digi XBee ZB SMT modules. These are different to the normal XBee modules because they are surface mount and additionally can interface to a host using SPI as well as serial. The backpack also features 3 LEDs for status signals as well as two FTDI headers (one for the RPi and one for the XBee), as well as being able to host a 'normal' XBee module (though of course you can't use SPI with those).
RPi Servo board
The Raspberry Pi Servo board by adent is a small expansion board for connecting up to eight RC servos and/or an ESC (Electronic Speed Controller) to the RPi. It will have a pass through for the GPIO port and it uses the serial line in it for commands. The interface is a simple ASCII-protocol via the serial port. One prototype exist so far. It also has five own GPIO-pins that is not supported by card firmware yet. Firmware will be released as GPL. Youtube clip of first prototype.
Raspy Juice Exp Board
Raspy Juice by 2-Watt Elements is an experimental board that supplies the Raspberry Pi with +5V through the GPIO header. With its wide input voltage range (6~23V), the buck regulator can be powered by batteries, 12V Adapters, solar, etc. Other features are the RS232 level console port, PCF8523 RTC, and an expansion ATmega168A microcontroller as an I2C slave. This latter MCU has interfaces of an RS485 port, an additional RS232 port, 4 RC-servo ports and unused-pins expansion header. The MCU firmware is updateable through the I2C interface. Hardware details are in http://code.google.com/p/raspy-juice/wiki/HardwareDescription
pyMCU is a python controlled microcontroller that works with the Raspberry Pi, connects to the USB port and uses the pyMCU python module and pyserial to control the microcontroller in your python script. You can communicate with i2c, spi, serial, and one wire devices, software configurable pulse in / out functions, has 13 Digital IO Pins, 6 10-bit Analog Pins, 5 10-Bit PWM Pins, blink LEDs, control servos and motors, read various sensors, built-in functions for sound generation: arbitrary frequency generator, misc. sound / tone generator, DTMF generator. PyBootloader - python script for uploading new firmware versions or your own microcontroller code. Check out some of the Tutorials and Examples
RPi CC1101-OneWire-Clock extension
The RPi-COC board from busware.de adds Onewire, a Sub-1 GHz RF Transceiver, RTC and an EEPROM mainly for smart home automation. The Onewire RJ11 port is driven by a DS2482 chip supported by 5V levelshifter and strong pullup. The DS1339-RTC is powered by CR2032 battery or on-board 60mF supercap. The 2Kbit EEPROM (at24c02) is mainly to hold board specific config data. The CC1101 Sub-1 GHz RF Transceiver is controlled by an atmega1284 to pre-process air data. It is connected to RPis UART and contains a avr109-bootloader for firmware updates. The popular culfw-firmware lets you interact with FS20, Homematic, Intertechno or similar actors and sensors - easily using FHEM-home automation server. In addition a secured 5V power socket for external power supplies is provided.