Getting Things Done adaptation that works with Thunderbird
Next Action is a Getting Things Done todo-list tracking tool. It's a personal database for your GTD info.
GTD Tiddly Wiki is a GettingThingsDone adaptation by NathanBowers of JeremyRuston's Open Source TiddlyWiki.
GTD - Getting Things Done - is a book by David Allen, giving a series of principles for managing the day to day tasks and projects we all have to do.