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.
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
Lucida Sans Mono
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.
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.