I am a GTD follower, or rather a GTD non-practicing believer. :p
One of the best things that GTD has taught me though is the power of lists. A flat collection of all the actions that you need to do to accomplish something. In the real, paper and pen world, I always keep my stack of index cards handy for jotting down quick lists, and for the software bits, i use sticky notes to keep track of short-term action lists. I still suck at keeping things in control though: my GTD collection proces is really a mess, but I still manage to make a little out of it. This is one of the the good things about the GTD sys tem: you can still benefit from it even if don't implement the whole thing. That actually ruins the whole idea, but oh well. :p (I'm really trying to start with my GTD system again)
That being said, and moving on with the topic of lists, i have also tried setting up online task lists. Web-bases lists has the advantge of being available anywhere you have internet access and some tools actually integrate well with my existing online workflow tools such as email and IM.
I have tried backpack, ta-da list, both 37 signals products, Google IG's to-do list widget and a some other web apps that I can no longer remember.
Tonight I started experimenting with Remember the Milk, a weirdly-named but actually clever and interesting web app. I'm still yet to master all of its features and bells-and-whistles, but the app actually seems really promising.
Need an online list? Explore the apps i mentioned above. :)