We've had a few occasional threads concerning this. My preference is for BBEdit (full version, not Lite... which is still good for free, but lacks the all important eyesight saving code-colouring). In fact, BBEdit is really all I use. For XHTML, CSS, PHP; pretty much most code that I need.
Occasionally I'll venture into Dreamweaver MX (dumped it about 6 months ago but still have a license for MX), for PHP (I'm not advanced, so DWMX's PHP handling can be helpful).
Other applications noted here recently are skedit (do a search on
www.versiontracker.com/macosx for skedit). Which I think is about $20 (£10). Very good, but I still prefer BBEdit. Also, you might want to look at CSSEdit for a very comprehensive individual CSS application.