I am interested in solving some mathematical combinatorial problems. Besides the Polymath project, are there some other online projects where I can state the problems, my approach towards a solution, and then to expect some people to help and collaborate in order to solve the problems and then together publish the solutions?
I think one of the best collaborative mathematical websites is here on Stack Exchange; take a look at Mathematics on Stack Exchange to see how many different mathematical questions are posted with various topics and many professionals and mathematicians also collaborate in this site so the user can easilly can learn many mathematical things, find many professional answers and become familiar with new mathematical topics as well. Even by just reading the questions and answers on that site, the user gets familiar with many mathematical topics. It is helpful indeed, at least for me. I strongly encourage you to take a look at that website and enjoy mathematics.