Tracks

Tracks is a GPL Rails application to help implement David Allen's "Getting Things Done".

I've installed it on my MBP and it looks great so far. You can tag tasks (and view only tasks for one tag), set up contexts such as work/home/misc and then view things grouped by context, set due dates, set tasks to only be visible after a certain date, view graphs and statistics about how long it takes you to complete tasks, comes with an API so you can write your own scripts to interact with it - it's very comprehensive.