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
- 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.
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.
On 2nd January 2012, CJE/4D announced that they have a real-time clock (RTC) expansion board in the works: 
Pi-Wire Homelabs are developing options for connection of a 1-wire bus to the RaspberryPi. Boards include SPI breakout connections for future boards.