I am a long-standing devotee of the
David Allen "Getting Things Done" (GTD) approach to life, as I first described in
this Atlantic article about him four years ago. We've become friends and stayed in touch since then too, which at least for me has been very enjoyable. Plus, since long before the Atlantic wrote about him he has been a loyal subscriber!
The GTD Way mainly involves habits of mind and action, but it also places a lot of emphasis on having the right tools, gizmos, and gimmicks to support those habits. Over the years I've used a variety of software to set up GTD-based systems on my computer. Ones I've liked include
Results Manager and
Chandler. The one I keep coming back to for my own purposes, more than a dozen years after I started using it, is the idiosyncratic but powerful
Zoot. Zoot is PC-only, and for that matter text-only (no graphics etc), but it runs flawlessly on a Mac under
VMWare Fusion.
Here are three more to bear in mind, with different strengths and idiosyncracies of their own:1.
OmniFocus (Mac only), from the same group that makes the excellent Mac outliner
OmniOutliner. This is the most straightforward of the programs I'm mentioning here. You set up Projects, Contexts, Actions, etc, straight out of the GTD gospel, and you go from there.
2.
ThinkingRock (PC, Mac, Linux), from a tiny firm in Sydney, Australia. Maybe I should move to Australia: apart from the obvious attractions, a disproportionate amount of "interesting" software seems to come from there. I've previously praised
Surfulater, for collecting and sorting material from the internet, and
Rationale (and its offshoot bCisive), designed to assess the strength and logic of arguments. ThinkingRock is from this same innovative tradition. It looks "different" (see below; click for slightly bigger version), it has a different kind of logic from the standard old PC-based program, but it also adheres very closely to the GTD vision and spirit. Like Zoot, it takes some adjustment and learning. (I still prefer Zoot to all others because it is almost infinitely configurable and adjustable.) But certainly worth checking out.

3.
MonkeyGTDWiki, which also is absolutely intriguing in the way it looks and works, and also clearly springs from the GTD tradition. The simplest way to explain it is as a browser-based personal Wiki, which you can configure to show your projects, actions, contexts, deadlines, and so on. Demo
here and underlying info about the TiddlyWiki engine
here. Because it's browser-based, it obviously works with any kind of computer. It requires more digging-in even than Zoot, but it has its rewards. Have fun. Also,
this post from LifeHacker discusses some other GTD systems.
Update: Discussion of a zillion, or more precisely 102, GTD-related programs can be found
here.
This article available online at:
http://www.theatlantic.com/technology/archive/2008/05/bright-side-5-interesting-gtd-software-including-for-mac/8013/