The Creator and Developer Advocacy group over at Unity have just launched a new initiative called Open Projects, a Unity lead effort to develop a vertical slice of a game that is open source and community driven. If you are looking to get experience at working with a team, or perhaps are a student looking to build up your resume, contributing to an Open Project could be a good fit. The intention is to create open projects for multiple genres, but initially they are starting with an action-adventure style game. The final results will be published (assumedly for free) on Steam, with all contributors credited for their efforts.
There is a bare-bones project in place now you can download from GitHub. In fact GitHub is central to the entire process, as this is where the project will be housed and where all code and asset collaboration will occur. In addition to GitHub they are coordinating the project in a dedicated Open Projects forum, as well there is a Contribution Guide on Google Docs available here. Project management tasks are managed here powered by Codedecks, an online PM tool specifically for game developers.