If you're familiar with Wikipedia, then their definition of a wiki would be a good place to start.
I like the idea of using a wiki to implement GTD, as it allows more flexibility than an outliner app. (These usually have some kind of tree-like hierarchy.)
I have just started using Instiki, and so far, I like what I see.
If you don't need to access your lists from multiple locations, or if you want to store them on a PDA, then Note Studio (search this forum for more info) could be for you (free demo, $50 to buy, IIRC).