One year back (on 6/6/12) I announced a beta version of TrueShelf, a social-network for sharing exercises and puzzles especially in mathematics and computer science. After an year of testing and adding new features, now I can say that TrueShelf is out of beta.
TrueShelf turned out to be a very useful website. When students ask me for practice problems (or books) on a particular topic, I simply point them to trueshelf and tell them the tags related to that topic. When I am advising students on research projects, I first tell them to solve all related problems (in the first couple of weeks) to prepare them to read research papers.
Here are the features in TrueShelf 1.0.
- Post an exercise (or) multiple-choice question (or) video (or) notes.
- Solve any multiple-choice question directly on the website.
- Add topic and tags to any post
- Add source or level (high-school/undergraduate/graduate/research).
- Show text-books related to a post
- Show related posts for every post.
- View printable version (or) LaTex version of any post.
- Email / Tweet / share on facebook (or) Google+ any post directly from the post.
- Add any post to your Favorites
- Like (a.k.a upvote) any post.
Feel free to explore TrueShelf, contribute new exercises and let me know if you have any feedback (or) new features you want to see. You can also follow TrueShelf on facebook, twitter and google+. Here is a screenshot highlighting the important features.
My Brain is Open 
Perhaps it is there, but I was unable to find on the site a license for posts. If it is not there, can I suggest that you include a statement to the effect that all posts are under some license that makes them freely-usable? That’d be great.
The license is mentioned at the bottom of every page. It is Creative Commons Attribution Share Alike 3.0.
Nice site! Would have been even better if one can subscribe/follow problems in a particular topic.