Recs.
Updated
This open source font family is derived from the Bitstream Vera family, itself close to the Microsoft core Web fonts (see Andale Mono).
SpecsUpdate
Pros
Pro Very clear distinction between similar characters
It's easy to distinguish between characters such as I, 1, l, O, and 0 in DejaVu Sans Mono.
Pro It's subtle, yet stylish and extremely readable, very easy on eyes and very effective for long development sessions
Pro Excellent unicode support
DejaVu Sans Mono has one of the most complete Unicode fonts available. This means you have access to a wide range of special symbols including mathematical symbols like arrows, operators, and special alphabets. This is useful for certain languages that require special characters like Agda.
Some languages allow using these characters optionally. There are editor modes that display characters like this without changing the underlying file, much like syntax highlighting. The Emacs modes for OCaml and Haskell are prime examples.
Pro Bold font is the same width as the regular weight font
The Sans Mono version is graphically close to Andale Mono (Microsoft core web font), slightly bolder, with the added bonus of the bold font being the same width as the regular one (unlike Andale Mono). It is a nice property with some syntax highlighting text editors.
Cons
Con Crowded bold styles
At size 12, in bold text, some letters bump up against each other too closely, greatly reducing legibility.
Con The "-" symbol is short
For example, when using the '-' symbol for borders, it's super short in this font.
Recommendations
Comments
Flagged Pros + Cons
Pro Available for all platforms
The font is available on OSX, Linux, Windows Vista, Windows 7, 8, and 10.
