When I'm not in Xcode, my preferred editor is Komodo Edit (with the optional Vim bindings turned on). It's the editor portion of their IDE and is free.
If you're married to the TextMate UX you should look at BBedit.<p>If you're open to trying an all-in-one I'd add Espresso from macrabbit and Coda from panic to my short list. Between the two I think most consider it a toss-up, although I slightly prefer the former.
I can't help but recommending Carbon Emacs. And probably someone else will come and recommend vim. I won't advocate more than needed, it is just awesome ;)