libgit2 is...

licensed under a very permissive license so you can use it anywhere
faster than any other Git library
written in standards compliant C99
completely multi-platform: Windows, Linux, Mac OS X, xBSD
compiled natively under all platforms (yes, even MSVC on Windows)
re-entrant, with sane error handling
designed with a solid and consistent API
available as bindings for all major scripting languages

WWW: http://libgit2.github.com/