Tim Brom of Calvin College built the Microwulf Supercomputer, that flips 26 gigaflops (26 billion double-precision floating point instructions per second) and cost $2,500 to build last year (and would only cost $1,256 to build today).
It consists of four microATX motherboards, each with a dual core CPU and 2 gigs of ram, all connected with an 8-port Gigabit Ethernet switch. The whole shebang also includes a CD/DVD drive and a 250 gb HD, and runs Ubuntu Linux.
I want one. Maybe in a slightly cooler case (like this), and with some, uh, dust protection (like this), and I'd be good to go. Ten points to Tim for the sweet computer, but minus two for having his Matrix poster rolled up in the corner.
One more design picture after the jump.