A good monospaced font will increase the readability of programming source code.
The fonts listed below are some open source alternatives to the default monospaced OS fonts (Courier, Consolas, Lucida Console, Monaco). The ideal font will depend on your monitor size and DPI, font size and whether you use Font Smoothing/ClearType.
Anonymous Pro by Mark Simonson - Full unicode and bitmap support.
Andale Mono - via Microsoft Core fonts.
Adobe Source Code Pro by Paul D. Hunt.
Bitstream Vera Sans Mono by Jim Lyles. A derivative of Vera Sans Mono, Menlo, is provided with Mac OS X. DejaVu fonts are also based on Vera.
Courier Prime - an updated version of Courier with true italics, designed primarily for screenplays.
Droid Sans Mono - via the Google Andriod project. Commercial versions are also available at DroidFonts.
Inconsolata by Raph Levien, similar to Consolas but works equally well with ClearType on or off.
ProFont and Sheldon - small bitmap fonts for programming.
ProggyFonts - small non-cleartype fonts.
Ubuntu Font - custom designed font with a very distinctive look and feel.
“The loveliest faces are to be seen by moonlight, when one sees half with the eye and half with the fancy” ~ Persian Proverb
Google Noto - Open source unicode font, Noto Sans and Noto Serif support 581 languages.