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DejaVu Sans Mono is an open source font family forked from Bitstream Vera v1.10 in June 2004. The goal of the project is to retain the aesthetic qualities which led to the success of the Vera fonts and adding much broader character sets, potentially culminating in a truly "universal font," usable in any language regardless of its alphabet. It is visually comparable to the Microsoft Core Web fonts, notably Andale Mono.
SpecsUpdate
Pros
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.