I just love the new Microsoft Consolas font that comes bundled in Windows Vista. "Pragmata" is also a very nice and professional font that is a bit expensive though, and misses support for many languages other than english.
For me, the definitive feature that distinguishes a simple monospaced font from a font adequate for programming is the slashed (or dotted) zero that separates it clearly from capital "O".
Being able to distinguish 1 from l is also a benefit.
I personally prefer Courier (although 11-12 points is excessive IMO). It's available in most systems, it's perfectly clear with or without ClearType and it has fixed width. Perfect.
I love Consolas at work but for Linux a great alternative is Inconsolata .. especially if you enable the smooth rendering
Kind of related: I've settled on ori1Uni, size 16 as my default font for Firefox. I find it makes non marked up HTML much much easier to read.