Inconsolata

This is my favorite monospaced font, as its:

1. Free - it can be used anywhere, on anything, no charge, (which can't be said for proprietary fonts that ship with other OS's)

2. Looks good on screen and when printed (normally or via fpdf/xetex)

3. Has 0's and O's that are instantly distinguishably from each other. See also the I, 1 and l characters.

4. Is valid. You'd be surprised at the number of fonts out there with broken leading, or are in some way corrupt to the point that FontBook throws a fit.

13 point looks particularly good in Terminal.app/gnome terminal or in [g|m]vim.

Highly recommended if you're looking for "your" font.

Highly recommended. IMO it beats other good free fonts such as DejaVu Sans Mono, Droid Sans Mono (has problems with zero vs Oh) and Liberation Mono. On Ubuntu is packaged as "ttf-inconsolata".