As general-purpose text editors go, TextMate on Mac OS X is probably one of the best I've ever used. Favorite features are the built-in spell-checking, solid macro support, and a clean UI that stays out of the way. It also offers syntax highlighting for many different languages out of the box, which is nice when because I work in HTML, Java, and Ruby a lot. Smart tabs generally work very well also.
There are only two things about TextMate that I don't like much: auto-pairing of characters (like quotes and brackets), and auto-indent when I don't want it. That being said, the behavior of both of these can be adjusted. I'll include tips on both of these in a little while.
With a free 30-day trial it's also hard to go wrong. I think the low purchase price is a good idea too; I've never gotten too many arguments from management on low-price purchases.
While I'm thinking about it, TextMate: Power Editing for the Mac may be a good supplement, but check out TextMate's help material first, it seems pretty thorough.