Most mudding clients will come with a font pre-set. Often this will be a thin, spidery font, or a blocky, brick-like font. Here is one endeavour to display a good selection of monospace (all the letters the same width) fonts that are easy on the eyes, and suitable for mudding.
Looks a lot like Monaco. Comes with various versions of Windows or MS Office.
Bitstream Sans Mono & Deja Vu Sans Mono
For most end-user purposes, these two fonts are the same. Free as in software. Available from here.
Comes with Windows Vista and MS Office 2007(?). Like a version of Courier New that doesn't make your eyes hurt and your spider lustful.
Courier & Courier New
Very light, typewriter-style fonts. Courier New is the default font for ZMUD and CMUD.
Lucida Sans Mono
This and SimSun are unusual in being monospaced fonts that also have proper serifs (the pointy bits on the ends of letters).
Comes with Mac OS X. Looks better in small sizes, and without anti-aliasing.
OCR A & B
This is painful. I don't recommend it.
This and Luxi Mono are unusual in being monospaced fonts that also have proper serifs (the pointy bits on the ends of letters).
Dina programming font. Pretty, free. Only comes in a limited selection of sizes, but they're all good.
Default MUSHClient font. Chunky, unpleasant. Comes with Windows.
Comes with most Linux distros. There's a Windows package floating around somewhere as well. Some of them are very readable.