ProFont is an excellent monospace font, created by Stephen C. Gilardi and Carl Osterwald, that clearly distinguishes between O and 0, as well as i, l, and 1. However, the existing font file has ligatures which make the font less useful in Cocoa apps under Mac OS X.
I used a font editor to remove the ligatures, and created ProFontX. If you'd like to try it, you can download the font here.
Note: this file is based on a version that predates the inclusion of the Euro symbol in the typeface.