CSSEdit has the most powerful and well thought-out interface I've ever seen for working with stylesheet properties, and its source-and-visual CSS editor is rightfully award-winning. If you've got to spend more than an hour a week in stylesheets, CSSEdit is the only reasonable tool for the job.

If you're more of a codemonkey, TextMate is the darling of both the Unix refugee camp that took shelter in OS X recently, as well as the Ruby and Rails development communities. It's sort of the best damned text-and-code editor ever produced for the MacOS, with the possible exception of the long-time heavyweight, BBEdit. The bundles are well structured, and support almost any language or variant you can think of.

If you're in the market for power editors, you could do worse than giving these two a spin. Of course, if you're the "all in one" sort, there is always Coda, by the boys at Panic. While it was slightly raw the last time I used it, if I hadn't already owned a license for TextMate and CSSEdit when it came out I'd have probably purchased it. It's improved considerably, and it is under steady development, but it is extremely web-centric (see the supported languages) and that might be a drawback for some people.