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
Basically any proportional font will do it for me, as long as it has good hinting, or is bitmap. I stick with the Lucida family of fonts for this. There has been research done though to prove it really doesn't matter what type of font you use, but the spacing of the letters and words. (See: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3075059/) --lf94
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/