OmniFocus is well worth a look
Back in July I posted my preferences were iGTD and Journler. While I still can recommend both as excellent choices, I have been working with the alpha releases of OmniFocus and really like what the team at Omni has been doing. The public beta is now available (official release is slated for early January) and there's a hefty discount if you purchase a license prior to the official release.
I don;t want to get into the free vs. commercial debate. I know the developer of iGTD ultimately plans to release a paid version but has stated that there will always be a free version of his app and I think that's admirable.
Also, after using Journler for a while, I've returned to Yojimbo from BareBones Software. Again - it's a commercial vs. free thing. journler is a very nice app but I don't need a lot of what it does and I like Yojimbo's integration with the OS.
All are worthy candidates for GTD on the Mac. YMMV so take advantage of the trial periods, experiment, and see which best suits your needs and work style.