Or a nuclear reactor, though I can't say I've seen one up close. Radiation and all that jazz.
Apparently this unusually-shaped monstrosity (the pretty kind, of course) is the prize of a competition held by Crysis fan site inCrysis. The competition demanded entrants guess the final system requirements for EA's recently released shooter.
The beast you see here is the physical representation of those specs; inside breathes a 3GHz Core 2 Duo and a GeForce 8800 GTX - a grunty combination in anyone's books - along with a pair of 150GB Raptor HDDs.
You can find more photos over at the official thread.
Dream Extreme PC Photos [inCrysis, thanks Brendan]