The VIA EPIA motherboard is pretty cool. Measuring only 17cm square it's got firewire, usb, video and sound all integrated. It seems to do an alright job playing DVDs, DivX, MP3s and being a file server. The CPU is either fanless at 533MHz ($AU214) or with a quiet fan at 800Mhz ($AU245).
A good review by Tech PC:
It looks like the return of the Mac cube in PC form with versions including an old Sun IPX, grey metallic ones and the ice cube. When will someone do the inevitable sphere?