This website allows for writing projects where multiple authors can write a book or short story together. I think this would be a wonderful team project to work on in groups of 3-4 (maybe more).
This is a really comprehensive, university-based math site. It is free, and no membership is required to access the resources, which are include help with math content, puzzles, discussion groups, and an annotated internet math library. There are sections for kids, teachers, parents, and researchers.