I wonder how quickly and to what extent online courses will replace residential colleges and K-12 schools. I think the potential is especially high for math and computer science. The natural sciences and engineering require laboratories, and in the humanities, class discussion and comments from a teacher or professor on written work are important. The breakthrough for online education will come when/if big-name employers decide that degrees earned online are credible.
So far my kids have used EPGY and Art of Problem Solving (AOPS). AOPS courses have weekly online classes lead by instructors where students can answer questions in real time.