Dreamweaver is still the de facto standard for professional web development, and the OS X version is basically identical to the Windows version. Another professional tool that's widely used is Adobe GoLive.
But if you're looking for something a little cheaper (i.e. free) both the Mozilla Suite and Netscape include Composer, a free HTML composer.
If coding is more your thing there are options too. BBEdit is hands down probably the best text editor around for OS X. Its freeware brother TextWrangler offers a slew of text editing features as well.
There's also SubEthaEdit (formerly Hydra) which by itself is a great editor but it also has the unique feature of networking over Rendezvous, for pair programming with two or more people.
Some other recommendations, offered by ATM forum members:
Taco HTML Edit