I've been using the ThinkingRock GTD application for several weeks now. I like the fact that I can run it on both my Mac and Windows machines.
However, there is a new player in the GTD application arena. The GhostAction is a Mac only app. and looks very promising.

One thing that is intriguing about the GhostAction is it's synchronization with iCal. I like that a lot. here is what they say about their connectivity with iCal:

Ghost Action features full two-way synchronization with iCal, which enables synchronization of your to-dos to any iSync compatible device such as a phone or PDA, so that you may access your to-dos even when you are on the move. Users, who don’t use a portable device, can achieve the same goal by printing their to-do lists.

And here is one of the better explanation of GTD from their site ;-)

Getting Things Done® (GTD) is a simple method for managing your to-do lists invented by David Allen that recently has become very popular. Everything you need to-do is listed as actions. Unlike many other methods Getting Things Done doesn’t emphasize priorities. Instead, actions are grouped according to the context they can be performed at, and the project they belong to. Possible future actions are ignored until they can be acted upon. This makes GTD stress-free, simple, and attractive—even to people who normally wouldn’t use a time management method.

I'm going to give this a try. Specially since I can get a free copy.

(via: Productivity Workflows)