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.