Fixed width, generally Courier, though that's as much familiarity as anything. There's an interesting discussion of the topic at the Typophile site, here: http://typophile.com/node/19653 with links to an assortment of candidates.
Fonts are subjects of Holy Wars, just like editors. As an ex-designer turned computer geek, I am highly amused by a lot of it. --DMcCunney
I have found the fonts Proggy Clean and Consolas very useful. Both available free online. --jcrites
Lucida Console and Vera Bitstream Mono are also pretty good. --rahp
Fixedsys a monospaced font. "It is the oldest font in Windows, and was the system font in Windows 1.0 and 2.0." Source: http://en.wikipedia.org/wiki/Fixedsys Fixedsys Excelsior is a ‘pan-unicode’ version of Fixedsys, it can be previewed and downloaded for free at http://www.fixedsysexcelsior.com/