Well, the VIA epia-based MiniITX systems are probably the smallest you'll find without going into PC/104 territory. They're soon to be trumped by the NanoITX boards, though (if VIA will EVER release them :|). In terms of Intel or AMD based systems like Shuttle XPCs, they're all about the same, except for the Asus that takes MicroATX boards (much bigger) and the other SFF system that has 2 cdrom bays (forgot who makes it).