I bought 'Dutch's CSS editor' around 2001. It was the best investment till now. I adviced to one of my friends to buy it. But it seems some other people owned it now. I couldn't find it there ...
I will disagree with all of you and throw a spanner in the works!
Visual Studio 2008 is the best (admittedly, if you can afford it or if your work will give you it). By far the best support for EVERYTHING! (just my view though... )
I've always used just basic editors for CSS, be it HTML Kit, Notepad, Notepad++ or even the editor end of Dreamweaver.
I have used Topstyle Light, but sometimes I find it's just overkill.