Textpow + Ultraviolet: TextMate Powered Syntax Highlighting for Everyone!
Textpow and Ultraviolet are a pair of interesting, new(ish) Ruby libraries that co-operate together to parse TextMate bundles and perform syntax highlighting. Textpow is a library that parses and processes the TextMate bundles (special files included with the TextMate text editor that specify how different types of plain text content should be delimited and colored). Ultraviolet then uses Textpow and performs syntax highlighting either as a standalone application or as a Ruby library.
Currently, Textpow can process syntax (tmLanguage and tmSyntax) and theme (tmTheme) files.. and you can obtain these from the free trial download of TextMate if you don't have them already (although, I imagine, non OS X users might be able to find other TextMate bundles and themes elsewhere, such as the Vibrant Ink theme downloadable from here).