Raspberry Pi. It's really an amazing PC. Not only it's really tiny, can run Quake III and play back Full HD video through HDMI, but it only costs $25 PC! Check out the video demonstrating Quake III at full speed.

And that's still an alpha board. The $25 Raspeberry Pi has the following specs:

700MHz ARM11
128MB or 256MB of SDRAM
OpenGL ES 2.0
1080p30 H.264 high-profile decode
Composite and HDMI video output
USB 2.0
SD/MMC/SDIO memory card slot
General-purpose I/O
Optional integrated 2-port USB hub and 10/100 Ethernet controller
Open software (Ubuntu, Iceweasel, KOffice, Python)

Pretty bloody amazing. [Raspberry Pi via TechCrunch]