well... the strange things we do to cool our PC's... *tuts*
There have been loads of custom cooling systems!
At the high-end, we see 2 or 3 stage cascades getting temps of lower than -100'C, with custom evap heads.
You see Custom waterblocks all the time (well I do) as they're really simple to make, and make well
I used to make my own HSF's for old P2's, Celeron's and AMD K6's just using 2Kg blocks of copper and a hacksaw, dremel and whatever else you might want... Had one in the shape of a skull, with two 40mm fans as the eyes..

was bloody heavy though, and the trick was to steal the mounting arm off of the stock cooler...
Might get back into that soon actually