Aviation Week online has good stuff...
The Aviation Now special section has some good information, including the transponder tracks of the doomed planes.
These tracks are eerie. In the case of flight 93, as you follow the track first west, then erratically southeast, you can almost imagine the battle on board the plane as the brave passengers and crew attempt to keep the plane out of terrorist hands at the cost of their own lives.
Aviation Week is a superb reference in this area, and will have all the good lowdown in future weeks on exactly what happened with these airplanes.