The Math Olympiad Organization Project is an open-source repository of problems collected from USA Math Olympiad Summer Program handouts (with permissions from the owners) past IMO Shortlists, and past USAMO's, started by Alex Zhu. It differs from other resources in a number of ways:

• It's prepopulated with a huge number of problems, organized by topic.
• Solutions are sorted by upvotes and downvotes, are always hidden at first, can show hints, and can be commented on. (Screenshot 1, Screenshot 2)
• Users can mark problems as favorites, problems they're working on, and problems they've solved, and access these problems (Screenshot)
• Users can search for problems by keywords, source, topic, difficulty, and elegance. (Screenshot 1, Screenshot 2)

Link: http://mathorg.herokuapp.com

I am currently maintaining this project, but am looking for someone else to take over. If you are interested in maintaining this project, please contact me at contact@alexzhu.me.

The Math Olympiad Organization Project is released under the MIT License.