The world's cheapest supercomputer, built by a Calvin College CS professor Joel Adams and student Tim Brom is very interesting. They built an 8 core Beowulf cluster using four motherboards and a gigabit network switch for less than $2500. The resulting machine has a price/performance ratio of $100/Gigaflop. That's just plain fun.
I think there ought to be a yearly competition of this sort for students. Who can build the fastest supercomputer for $2500?
Update: here's Joel Adams' website