If you're usually at the same computer this may not apply to you, but I have an iMac at home, a PowerBook G4, and multiple machines at work running Linux and/or Windows.

Since I've got a ton of machines and platforms in use, I run most of my stuff on the Web. Gmail, Google Calendar, that type of thing. I use simpleGTD for my project, task, and next action lists.

Best of all, it's free! I like it even better than RTM, because it's specifically tailored to use the GTD system.

First of all I would make sure that you really need any programs. I tried a lot of them (Omnifocus, Things, Midnight Inbox) and most of the time I came back to using TextWrangler and text files. Then using the grep command to retrieve contexts and that’s it.
It works for me as I think most of these apps lack something in some department, for example, my favorite of the bunch is Things (even though it’s still in beta) but you have to enter the “editing mode” to see the notes. Basically it all came down to speed, the fact that a text file is simple to navigate with the keyboard, and the fact that I didn’t fill constrained to fill in the tags or notes or titles in a particular fashion, and I just input how I see fit.

If you are interested let me know and then I can outline my “system” in more details.

Good luck! Finding the right system/application is the most important thing.

EDIT: A little question, do you need an online function/app or something that syncs with your iPhone? I don’t know about the iPhone (not available in the Netherlands since all providers switched to HDSPA already) so it would be cool if you could tell me what you can sync on it and not or if you want an online solution so you don’t need to sync at all. Cheers.

(1) Focus on reliability. The richness of the interface means nothing if you can’t count on it as a reliable system. Likewise for internet-based systems - the one day the servers go down or you can’t get reception, it ceases to be a trusted and reliable system.

(2) Focus on apps that help you learn the GTD habits and don’t just list your stuff in interesting ways. The apps that will make the biggest difference are those that help you get things done and not just organized.

Those two criteria are what led me to develop my own approach to GTD on the mac. Check out the backstory and see what I say about “Falling Prey to Temptation: Total Reduction and the ‘Perfect’ Program” and “The Downside of Lists and the Power of One-at-a-Time.”

There is also this comprehensive review of GTD apps - the most comprehensive to-date.

I've just reviewed the 4 top apps on my blog, Put Things Off. Check it out: http://putthingsoff.com/index.php/osx-task-manager-showdown/

Don't forget to subscribe to PTO if you're interested. :)
http://feeds.feedburner.com/putthingsoff/