Like any over-committed technologist who participates in multiple organizations, projects, hobbies, and just life in general - it's easy to obsess on finding the best task tracking tool. I used to be a dedicated user of iGTD, but somewhere in the upcoming iGTD2 release the developer ceased releases and progress disappeared. Since then, I've considered Things, OmniFocus, and even simpler mechanisms like TaskPaper or hosting my own with Tracks.
None of these options really clicked with the way I wanted to organize my projects and tasks. In addition, their ability to syncronize between computers, or mobile access was limited. At best it required MobileMe account, very expensive iPhone clients and still syncronized to a single machine instance.
When Remember the Milk released their free iPhone client (requires Pro version) I went back and checked the progress of this project. I was very impressed with how far it had come from merely a simple web task to a very simple, and incredibly flexible, web-based task manager. The most powerful capability here is that my task list is decentralized and can by accessed, queried, synced, and viewed transparently and independent of interface.
RtM has embraced and followed through on the paradigm of bringing the tool closer to the user, rather than forcing the user to come to the tool. Look over the very long list of official services, GMail gadget, mobile clients for iPhone, Blackberry, Windows Mobile, & plain mobile browser. I can input new items via Twitter (which means via SMS or any Twitter client is now a task manager), Email, or even Quicksilver. Putting tasks into Remember the Milk is as easy as the thought that created the todo and I can easily flow it out of my head and into a safe tracking system. I even made a Fluid app of RememberTheMilk.com which gives me a Desktop-like app.
Sorting and fulfilling tasks follows the common GTD methodology. In addition, you can setup locations and attach tasks to these locations. Like other mobile task managers, with the iPhone app you can then lookup nearby tasks or at a specific location.
Remember the Milk even has an API, although you need to sign up for it and verify you're using it only for non-commercial use or explain what your commercial use is. I'm thinking I need to do some integration of tasks with my home automation system as well as task completion trending.
I think that Tracks is still a good option. It's open-source and written in Rails - so it's easy to extend and modify. Building on Twitter or Mobile interfaces would be straight-forward and mean that I could have complete control and privacy of my items. Maybe I should put that on my "todo" list:
d rtm Extend Tracks with RTM functionality
Note: Kumbuku Maziwa means "Remember the Milk" in Swahili. This was the first phrase I learned as a way to move beyond "hello" while we were hiking for several hours in the Ngong Hills outside Nairobi. The locals thought I was super-silly for asking how to say Kumbuku Maziwa, but now I'll never forget the phrase.