One of the sites on the OER list is mathalicious. Wow! I can't believe this stuff is out there. If you are a classroom teacher this is just too juicy. As Alan November once said to us, "This stuff is low hanging fruit!" These resources "can" fit so well with what Dan Meyer, Paul Lockhart, Conrad Wolfram, Alfie Kohn and many others are saying about learning math. These multi-media activities are truly impressive, engaging, creative and are aimed at middle years to high school students.
Notice I said can fit so well. If you take a look at the activities they are very engaging and well put together. An example is the tunnel digging activity for ratio and proportion. If you take a close look it is really just traditional problem solving questions put in an engaging way. All the information is given, as Dan Meyer says, the way is paved for them to find the answer, no deep thinking is needed about the processes involved. For my taste, the questions give too much information for them to be process oriented, creative problem solving opportunities. For example, the simple question I would ask is "How much dirt would have to be removed to dig the tunnel?" I would not mention volume, I would not give numbers. As Paul Lockhart says, "let them struggle, then give them some technique, but not too much".
They also give links to favorites of mine like HippoCampus and the Khan Academy.
Among the other OER resources listed are: