Ultra low-cost computer the size of a credit card. Also check the Raspberry Pi SE: http://raspberrypi.stackexchange.com/
The Raspberry Pi is an ultra low-cost computer ($25 for Model A and $35 for Model B). Features include:
- RCA (composite) and HDMI video out
- 3.5mm and HDMI audio out
- 256 (Model As and old Model Bs) or 512 MB (Model B rev. 2) of RAM
- Broadcom BCM2835 system-on-chip (SoC)
- 700 MHz ARM11 processor
- VideoCore IV GPU
- SD card used as the boot device
For more information about the Raspberry Pi, please visit the Raspberry Pi FAQ and the Raspberry Pi SE.