One of the most impressive PC mods I've seen over the years was a running system submerged completely in a fish tank full of mineral oil. A pump circulated the non-conductive liquid through a radiator to keep things cool, and the whole setup looked freakishly awesome. Sadly, the setup wasn't all that practical, which is probably why we haven't seen much of this so-called immersion cooling used in the PC world.
A couple of years ago, Hardcore PC tried its hand at producing an immersion-cooled desktop fit for consumers. We haven't heard much from the company since. However, its website does list desktop and workstation variants in addition to a blade server.
Immersion cooling dabbles in the sort of extremes we tend to appreciate in the enthusiast community, but its real appeal may lie in data centers. Green Revolution Cooling has deployed immersion cooling in several server environments and posted pictures and videos for all to see. Interestingly, the firm claims that a 100kW system it installed for Midas Networks has resulted in a 50% reduction in overall energy costs for the Austin-based ISP.
Green Revolution's intro to fluid submersion offers some insight on why immersion cooling works so well. The company's mineral-oil-based GreenDEF coolant is said to hold 1,200 times more heat by volume than plain old air. GreenDEF's ability to transfer heat is said to be significantly better, and the liquid doesn't need to be at a substantially lower temperature than the components it's trying to cool. Thanks to Slashdot for the tip.