mini-itx
It's a really small motherboard form factor. They have PC's that use those motherboards that fit in a cdrom drive bay and run off the 12v case power, and even include a cdrom. Theoretically, you could put one inside a Mac and network it to the Mac it is inside of. Like a parasite, or something...
Anyway, that's a nerdy site where people use the small motherbaords to make all sorts of goofy small computers that look like a bread box, teddy bear, old Mac, etc. They're pretty cheap and make fine non-gaming linux based computers so it makes a great hobby for people with lots of time and the desire to do such a thing.