Ranked in these QuestionsQuestion Ranking
Pro Easy on user's eyes
Monaco's rendering is similar to that of Consolas but slightly more playful. Each character seems to be a tiny bit larger when compared to Consolas, yet code takes up the same amount of space.
Pro Good differentiation between clashing characters
The curly braces are easily distinguishable from parentheses, "0" is distinguishable from "O", and "1" is distinguishable from "l", to name but a few.
Pro Excellent legibility at non-antialiased small sizes
This font shines for legibility at non-antialiased small sizes. It was originally designed as a 9 pt bitmap font. This is excellent for when maximizing on-screen code is a priority.
Pro Small file size
Even if you are struggling with memory, this font is no concern. Its ttf file is just about 50 kb.
Con Only comes with Mac OS X
While it generally only comes with OS X, you can try here for installing on other platforms.
Con Ability to select different anti-aliasing grades was dropped
Before OS X Snow Leopard, it was possible to apply varying degrees of anti-aliasing to the font. At present, the previously "medium" anti-aliasing option is the only choice.