Simple text editor that supports highlighting and font colours
TextEdit is part of Mac OS X. You can find it in the Applications folder.
Use TextWranger. It's full of features (including syntax highlighting) and best of all it's free.
I'd vote for either TextWrangler (as has already been done) or TextMate. Both are terrific, but I love the TextMate UI, and I love even more that it's free and open source now.