Android has a useful GTD (task mgr) app, 'Shuffle'. Can't find a direct link to it but 1.4 is awesome in that it syncs to the cloud via Tracks which is an open source GTD web app. Create an account at http://my.gtdify.com and link it to your phone. Very nice.

Useful things Shuffle does:

1. As for all GTD apps, it has a context for a task so when you shift contexts you can filter tasks based on that. For instance I have a context 'at home before work' which I check before I leap out the door. 
2. Also has Google calendar sync -- it can create appts in your calendar. Although current version doesn't allow you to select the account. 
3. Its all free and open source (Shuffle | Tracks)