I've been playing with Remember the milk for some time now, I thought with my n800 it would be really useful...BUT... Actually I've found that the firefox extension they have released has really upped my usage, as soon as I can get my tasks synced with my n800 I'll definitively be upgrading to their pro version.
I've recommended this plugin to a few friends and they love it, so if you're looking for a new personal task managment solution then this is for you!
Remember The Milk - Services / Remember The Milk for Gmail
Remember The Milk for Gmail is a Firefox extension that allows you to manage your tasks in Gmail (complete, postpone, and edit tasks), add new tasks (and connect them with your emails, contacts, and Google Calendar events), automatically add tasks for starred messages or specific labels, and much more!
My firefox page is now back online, so check it out for a list of other useful plugins/addons.