It's funny you should post this. Just recently (as in this weekend) I stumbled across demerphq's homenode, which includes a link to an informative site listing fonts for programmers.

It's something I'd never really thought about before. I had been using Courier all along, and it never crossed my mind that there might be better suited fonts out there. About my only requirement for a font was that it be fixed-width!

So since Monday, I've been trying out a new set of fonts called Proggy. If I had to pick today though, I would go back to Courier, but I will wait to see if this font grows on me. I do like that it puts a slash through zeros, clearly distinguishing them from the letter O. That's about the only complaint I have with Courier.