Ready-Set-Do! is a full GTD-implementation for the mac. It now comes with three Quicktime movie tutorials that give a quick overview of the core GTD habits (great even for non-mac users!).

http://homepage.mac.com/toddvasquez/apps

The advantage of this approach to GTD on the mac is that it comprehensively integrates all of one's files and programs into the GTD workflow without locking users into a program that may or may not have long-term future software support.

Once installed just choose one of the following from the Ready-Set-Do! folder in your dock:
1. Empty Your Head
2. Get INBOX to Empty
3. Get Some ACTIONABLES Done
4. Get Clear on PROJECTS
5. Follow-Up WAITING FORS
6. Get Some READING Done
7. Do DAILY REVIEW
8. Do WEEKLY REVIEW
9. Make Action Lists?
10. Ready-Set-Do! Help?

The site also has numerous other Xtras on productivity you may find useful:
? 10 Reasons Why Actionables Linger
? Pruning Your GTD System Checklist
? Why It's Not Getting Done Checklist

If you would like to try Ready-Set-Do! out (or you know others who would), I would greatly appreciate any feedback.

Sincerely,

Todd V
Ready-Set-Do! Creator