I love DejaVu Sans Mono:

http://dejavu-fonts.org/wiki/index.php?title=Main_Page

Which is a variant of Bitstream Vera Sans Mono:

http://www.gnome.org/fonts/

However, right now, my new favorite font is Menlo that came with Snow Leopard. It's another variant of Bitstream Vera and it seems to work much better at smaller font sizes. Which is great for limited screen sizes (such as 1280x800 on a smaller laptop).