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.
43 Folders is Merlin Mann’s site about personal productivity, life hacks, and simple ways to make your life a little better.
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.