I use Topstyle. A "lite" copy of this came with the version of Homesite I use, but I also have version 2.51 installed over it (which I got for free off a magazine CD). There are a number of free and freeware (I'm very keen on freeware) CSS editors out there. The ones I found after a simple search were (details added where I've used them)...
Amaya (free W3C's Editor/Browser) http://www.w3.org/Amaya/
This is an odd combination of browser and editor, which runs a bit slow. It's open source however, and if it works under Amaya, then it's probably "official". I haven't actually used the CSS component, but apparently it's there.
Style Assistant (freeware) http://www.styleassistant.de/
Not sure what to make of this. The website is in German which I don't speak, and the interface looks very forced.
WebAttack Freeware (Cascade DTP, Balthisar Cascade, xStyle) http://www.webattack.com/
Web Attack's collection of Freeware Style Editors.