Whether you're applying the GTD methodology or just trying to survive in our overcomplicated world, there's all sorts of loose bits of information you need to store and track. The problem with all this information from a GTD perspective is it tends to get mixed up both in your head and all on those little bits of paper with actual actionable tasks.
About three weeks ago I came across a tool that's easy to use, works on Windows, Mac and Linux, works in whatever browser you use, and to boot, is free. it's called TiddlyWiki, written by Jeremy Ruston.
you may have heard of wiki's: online applications that let anybody write and edit entries, like Wikipedia. TiddlyWiki is the same sort of application, except it all lives in a single HTML file you can download to your PC or Mac and open and use like you what it would a Word document. You store information in what are called Tiddlers - and you can tag this information, add them to the menu, use it to go to URLs you're interested in and more.
I'm using TiddlyWiki to store all of the planning documents for my next major micro-ISV application -- decisions about what the software will do, ideas for how to implement different features, narratives of how people will use the software, URLs that I want to take a look at at some point and more.
The beauty of TiddlyWiki is it takes absolutely no effort to start using it. Later, there are online groups, plug-ins, special formatting tricks and more that will reward you for the time you spend.
Since the entire application is one HTML file, you could put it on a server you control and can password protect, stick it in My Documents on your laptop and open it from a bookmark in Firefox, or put it on a USB key and take it with you. Since I primarily use two PCs here -- 1 for writing and communication, the other for development -- I'm using Microsoft's Windows Live foldershare.com free service to synchronize two copies of my TiddlyWiki.
Separating your reference information -- such as project plans -- from your tasks and next actions is a critical part of making GTD work. If you're looking for an solution that's easy, robust and free, definitely have a look at TiddlyWiki.