Official website – www.clearfont.co.uk
Latest version – r425xx
ClearFont2 is a simple, popular addon that changes the fonts used by World of Warcraft to a much clearer typeface.
It comes with several font style packages, which are easily switched.
Beyond that, with minor changes you can select any other font that you have, serif or not, fixed-width or not.
See http://www.wowace.com/wiki/ClearFont_2 for more info!