Odysseys2sense is a fairly new game-like forum in which anonymous contributors gain power by making posts that are rated highly by other players on criteria such as completeness, accuracy, helpfulness, and civility.
Several current forums are focused on mathematical topics that might be of interest to you and your students. Each forum consists of challenges, which in these cases are mathematics problems with a twist. In fact, college students sometimes find even the elementary- level problems challenging.
These forums (Odysseys) were set up for various courses, whose students are responding to the challenges. You and your students can freely read and review those responses and participate in the general discussions of them. I personally find the process, which encourages critical thinking, entertaining and very informative.
You can also apply to set up forums for your own courses.
After free and private registration, you can find mathematical Odysseys at
Copes: Problems in probability and statistics Copes: Conic sections Copes: Problems in secondary geometry Copes: Problems in secondary mathematics Copes: Problems in middle-school mathematics Copes: Problems in elementary mathematics Schield: Statistical literacy
There are other forums there as well, concerning pedagogical issues as well as a wide range of social questions. Some of these are more developed than others.