wePapers is an interesting idea. They bill themselves as creating the world's biggest study group. You can upload and download notes, presentations, articles, etc - virtually anything you'd like to share. Here's a link to a Common Derivatives and Integrals cheat sheet. Pretty nice - typeset, 4-pages; seems pretty comprehensive at first glance. You can even embed documents (see below) - great for an online or hybrid course.
This will be pretty handy for my Dynamics class this semester. If our documents, our classes, virtually anything you can think of is going into the cloud, why not study groups. I love it!
WePapers helps students and others share and expand their knowledge. You can find and download the papers and documents you need in a matter of seconds, discuss them with others, or just mess around.