The Mozilla Project has released
Camino 1.0, a
Mac OS X web browser based on the Gecko 1.8 rendering engine and the Quartz GUI toolkit.
Camino is free for Mac OS X 10.2 through 10.4.
It supports pretty much all the technologies that Mozilla does: HTML, XHTML, CSS, XML, XSLT, etc.
Mac OS X 10.2 or later is required.