I've been using CSSEdit for the last few days, and it's got a really nice interface. There is a left-hand pane which is the equivalent of the popup menu for functions in Textmate. And crucially, there is a live search box at the top which filters that pane, so you start typing, say 'news', and it shows all selectors with a class or id containing that word.
I personally think that would be a great feature for Textmate to have, perhaps in a drawer on the other side from the Projects drawer. The popup menu is pretty unwieldy (IMHO) with big documents.
--- Bruno