For a web development class, three of my good friends and I teamed up to build a site dedicated to collaboration on problem sets (PSets). Users were able to log in and find or create groups to work together, improving productivity and meeting new people at the same time.
We used Django to implement our ideas, because Python. Even though the site is now offline, along the way I picked up about SSH, Unix, virtual machines, Python, SQLite3, HTML, CSS, github, and the Django framework. Possibly even more valuable, though, were the brainstorming sessions we had to develop our product and try to come up with an idea that everyone would use. My collaborative skills improved dramatically due to this project.