rchrd pointed that a Cray Y-MP EL is freeely available for online access. This was one of the first mini-supercomputers - an idea that Cray Research played with fifteen years ago. The computer was a computing monster in those times (four processors, 1 GB main memory, a little bit less than one GFlop) but it is still an interesting design today.

http://www.cray-cyber.org/general/start.php