JEdit,
http://www.jedit.org
It's 100% JAVA, so it's cross-platform. It also has an awesome plugin architecture that allows it to be extended to do billions of useful things. JEdit comes with builtin syntax for tons of programming languages and a lot of useful programming tools.
If you are looking for a pure Windows program, EditPlus is it. It totally rules. Get it from download.com
SpaceAce