I have been playing with different GTD apps and methods for quite a while now on the Mac, and I have to say I really like this implementation. Plays nicely with Quicksilver, iCal and Address Book. The developer is very responsive and open to suggestions.

There are other good free GTD solutions out there (Frictionless and kGTD come to mind) but this one really shows promise and fits with my personal workflow style. YMMV since these types of productivity organizers come down to your own personal taste.

BTW, up to 1.3.3 on the website now....