Student annotated video of lectures
Today many universities are doing video of their lectures, and making it available on the campus LAN (or older campus cable TV.) In some cases students are not going to class, but many just find it a useful addition.
I suggest an application where students, while watching the lecture, could press keys on their computer synced in timestamp with the video. They don't need to be online, they just need a modestly good clock. Buttons like "This is important, review this for the final." Or even comments like "I already know this" and "I'm lost."
Students might use the timestamps themselves to build a "best of" video of the lectures, since you could not possibly watch all the lectures to review for the exam. The combined votes of students could be merged to produce a consensus vote on the best and worst parts of the lecture.
The professor could even review these things to see where the students are getting lost, what material they think is most valuable etc.
Of course this could also be done with plain audio of the classes but video would show the course materials and blackboards.
Perhaps one student in the class might take it upon herself to edit together a study video for others to use. They could even charge for it if it were really good.