Shameless plug, but I've just launched
zenlists (
http://zenlists.com) - a flexible webapp that allows you to maintain lists of items of any sort.
I use it for GTD - as the essence of GTD to me seems be lists, lists, lists.
I'd appreciate any feedback, it's still a little rough around the edges, but I wanted to release it early to see if it is of use to anyone else.