Tracks - GTD application on Rails
The installation of Tracks is unfortunately not for the faint of heart; if you're someone that would shy away from installing a web server on your local machine, or don't have access to a Ruby-enabled web hosting account, installing Tracks probably isn't for you. If you are, there are a number of sets of instructions available.
For the rest of us, our best bet is to find a hosted solution. Since Tracks is by its very nature multi-user, it appears to have been a relatively straightforward task to build a hosted solution, and allow users to pay a monthly fee for access.
My favorite of these hosted offerings is called GTDTracks. GTDTracks is extremely affordable, at $1.49/month, and they offer regular backups and upgrades. I'd much rather about getting my stuff done than messing with upgrading a web application that I know very little about, so this option appeals to me a great deal. Plus, they take care of what is arguably my biggest fear with online hosted services, which is the question of how to get your data out if you decide to move on to something else. GTDTracks has a policy of allowing users to cancel their account at any time with no penalty (so you're not on the hook for more than a buck and a half at any given time), and you can take your data with you, exported as a plain text file that can be imported into another installation of Tracks, or simply mined for the data it contains.
Using Tracks is very straightforward - simply create Contexts and Projects as needed, then create Next Actions (essentially tasks) and associate them with the project and context they relate to. Due dates can be set through a convenient calendar widget. One nicely done element of the application is the Done page - one way to get inspired to get things done is to see what you've already accomplished. The Done page shows items that were completed in the last 24 hours, last 7 days and last 28 days, as well as a link for everything older than that.