Firefox is one of the best browsers available and is compatible with Mac OS X 10.3 and higher. It supports tabbing, extensions, themes, is fairly security-oriented, and has one of the best times (program load and page load) around. There are a few known bugs in it, though they should be fixed in a release later this month. The next best option would be Opera, which has the same functionality and a more interesting layout but little third-party support. Aside from that, you're pretty much limited to Safari and IE (support for which was recently ended on the Mac platform), as the Mac is still a minority operating system compared to Windows.

Hope this helps,
John