I'd probably have to choose the following:
- Bitstream Vera Sans is a free font developed for the GNOME project but is available on other platforms (Download).
- Consolas is a relatively new Microsoft font that is installed in Vista or available as a separate Download. It is a very clear and compact monospace font and is being used by more developers.
- Dina is a great monospaced font and, for clarity, it is hard to beat (Download). It is available in 8, 9 and 10-point text sizes and looks awesome on any system.