Tea Cube & Space Cube
The SEMC5701A (Tea Cube) has a 333Mhz CPU, 16MB of flash memory and 64MB of RAM. It is tiny: 52mm x 52mm x 45mm but it still manages to pack a lot of ports including 100Mbit Ethernet, USB 2.0, speaker, microphone and serial port. The video port supports up to 1208x1024 resolution with 65K colors.
Any smaller and you can start wearing your PC as jewelery.
Via Epic Empire