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If you want free, TextWrangler is going to be one of the best options. If you don't mind paying, BBEdit or TextMate are two of the best.
 
It's already been mentioned, but I use Smultron for PHP personally (TextWrangler for more or less everything else). Can't tell you exactly why, I just sort of like the way it feels (and hey, free to try!).
 
Master vi and you'll never need to learn another editor. I even use a vi plugin when editing code in Eclipse as I find it much more efficient than ever touching the mouse. You can learn enough vi in 10 minutes to use it effectively.

You can download a graphical Mac OS X version of vi from here: http://www.vim.org/download.php#mac It is also built into OS X at the command line.
 
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