Guitarbots.comI signed up and started messing with is a couple weeks ago. Pretty interesting, the way they lay out tabs takes some getting used to, but after a while it's pretty intuitive. My only gripes is that it doesn't show stuff like hammer-ons, pull-offs, slides, and harmonics, and so far the song catalog is pretty limited. But you can upload your own guitar pro files (haven't tried that yet)
For beginners I can see it would be really very helpful getting started. For expert players not so much, but it still can be challenging and fun to play with.