RPi Expansion Boards

Introduction
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.

GertBoard

 * 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.

rpi_serial
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.

Piface
Piface is simpler than the RPi Gertboard (it's aimed at less experienced users), but also intended to connect to PCs and Android to be platform agnostic

CJE/4D RTC
On 2nd January 2012, CJE/4D announced that they have a real-time clock (RTC) expansion board in the works:

Pi-Wire
Pi-Wire Homelabs are developing options for connection of a 1-wire bus to the RaspberryPi. Boards include SPI breakout connections for future boards.

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.

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.

PiDuino
Youtube video showing prototype board

Pi Plates
Announcement Prototyping plate announcement

RPi Protoboard
RPi Protoboard is a simple prototype board for the Raspberry Pi another link to the same project?

RPi "UserPort"
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.

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 RS232 Breakout Board
RPi RS232 Breakout Board is a simple RS232 breakout board for the Raspberry Pi

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.