Windows/Mac/Linux: Doit.im is a free, attractive, cross-platform task management application built on the principles of productivity guru David Allen's Getting Things Done. It syncs to the web, it's easy to use, and soon it'll sync to your phone too.
(Click the image above for a closer look.)
Once you install the app and register for an account, you can start adding items to your inbox, creating projects and scheduling your tasks. A lot of the organisational tools will be familiar to GTD followers: notice the Projects and Someday inboxes, as well as the next actions section for arranging your tasks. You can easily drag and drop tasks into any bucket across the application, and while I ran into a snag on my first run, everything seemed smooth sailing after I started it up again.
Doit.im is a free download for Windows, Mac OS X and Linux, requires Adobe AIR. The developer claims mobile versions for the iPhone, Android, Windows Mobile and Symbian devices are underway, which would be a real boon for accessing your tasks anywhere, especially since they'd sync with your desktop. If you're looking for a new task manager and you're keen on GTD, Doit.im is certainly worth a look.