The Mac platform has an embarrasment of riches with regards to Getting Things Done applications, but the grand-daddy of GTD on the Mac is Ethan Schoonover's Kinkless GTD, a set of AppleScripts built on top of Omni Software's OmniOutliner Pro. Omni finally seemed to realize the significance of this and sought out help from Ethan and other GTD-folk like Merlin Mann in developing their new GTD application OmniFocus. Now, a year later, they've actually hired Ethan. After an extended private beta and lots of teasing glimpses, Omni has finally announced the initial public beta and introductory pricing.
You can download the public beta now and if you like what you see you can pre-purchase it for 50% off the release price. So if you buy it before January 8, 2008 it's $39.95, afterwards it's $79.95. Given the many (cheaper) GTD applications out there it will be interesting to see how well received OmniFocus is. But given its pedigree, one would expect it to be a leading contender out of the box. If you want an introduction to OmniFocus they also have a 15 minute quick start video available on their main page.