If you've not heard of Khan Academy yet, where have you been? No, seriously, where? I've been using Khan Academy to review for the middle school science Praxis exam, and I have to say that I'm impressed. Sometimes I wish the pacing was a bit faster since I'm just reviewing instead of learning, but since it's geared toward learners I can't really complain. I also wish there were exercises and questions for topics other than math (though the math ones are great). There are some good tools for teachers available on the site, as well.
Khan Academy is a good way to try flipping for upper elementary, middle, and high school classrooms. That is, instead of lecturing at school and then assigning homework problems, have the kids watch the videos at home so you can be there to support them while they work through problems. I'm really interested in flipping, since my skills are much better put to use supporting students while they work through their understanding rather than lecturing. I've also seen teachers make their own videos for their flipped lessons, so I'm hoping someone starts to put together the best of the best from across the web so we have a whole library of lessons to choose from.
Remember my Livescribe Pulse smartpen (the new version's called the Echo)? You can use that to create what's called a pencast, which has some similarities to what's going on at Khan Academy and it means you have control over the content. It's not as fancy, but if you don't like relying on someone else's content (or your school won't allow you to do so), it's one way to make your own.