OmniFocus is a great app for tracking and improving your productivity. It's currently in beta, and you can download a free copy to test/play with here. The Omnigroup is offering a special discount to beta-testers - if you pre-order before January 8, 2008 you can get OmniFocus for $39.95 as opposed to the release price of $79.95.
 
I grabbed my beta copy and have been tinkering around with it. What I like is that it doesn't force you into the GTD methodology - you can use it as a fancy to-do list tracker, and when you're ready to dig deeper the extra functionality is there. It also syncs with iCal, and can watch your email for incoming tasks that you mail yourself - for example, if you're at work and think of something to add to your task list, just email it to your home account. OmniFocus will pick it up the next time you check your home email and add it automatically.