The Raspberry Pi is an ultra-low-cost credit-card sized Linux computer. I.e., $35 buys you a small device with 2 USB ports, ethernet, HDMI, and analog video and audio out. The device has 512MByte SDRAM on board and comes equipped with an SDCard slot for 'external storage'. There are several Linux Distributions available for the RasPi and what you can build with it, seems almost limitless.
The Raspberry Pi, a general purpose computer, a headless server, an embedded system:
Creating a Raspbian (optimized version of Debian) SDCard to boot from, containing LXDE, Midori, etc.
Cutting the cord, using a small WiFi USB-dongle with the Raspberry Pi
TightVNC - Providing a GUI even when running the Raspi as an headless server
Using the Raspberry Pi as an IP Web Cam Server
What's happening at boot and how would a tiny "Hello Word" distribution look like
Using a RS-232 Serial connection to talk to the Raspberry Pi
Using the Raspberry Pi as an truely embedded system. Hacking with SPI, light sensors, LEDs etc.