I would agree with VCS, preferably distributed and centralised. It does mention GitHub and Kiln (Mercurial) "Work on a small piece of a large system (codebase

I would agree with VCS, preferably distributed and centralised. It does mention GitHub and Kiln (Mercurial) "Work on a small piece of a large system (codebase), read and understand existing code, track down documentation, and debug things. Notes: Github is a great way to read other people’s code or contribute to a project. Online Resources: Github, Kiln"