CSSEdit makes the process of testing out CSS design painfully simple and at $29 it's hard to pass up. It's got all of the helpers to guide you through CSS settings - pretty much expected for a decent CSS editor. You can use a dialog based approach or edit the CSS source directly in a full text editing surface that has lots of helpers.
The part that made me whip out the credit card though was the Live Preview browser. Point it at your HTML and start making changes in your CSS - they are reflected immediately in the preview browser. In addition it has an X-Ray feature that allows you to select any element on your finished HTML page and see the CSS classes that lead into it.
Great stuff - until you realize that you can point the preview browser at ANY page on the web. It sucks it in and allows you to modify the CSS on it. Sure, it's only a local render but it's a great way to check out how a nice looking site leverages CSS.
If you do any CSS work at all you should check out CSSEdit.