Version control repository
Linden Lab doesn't yet provide live access to a central revision control repository for the viewer source code. However, this is being considered.
Choices to consider
- Subversion - default choice, and the obvious "safe" choice. Pros: familiar to us, used by a lot of people Cons: very centralized model; need to give people access to facilitate effective collaboration
- Mercurial - bleeding edge choice. Pros: Distributed model makes it easier for anyone to distribute patchsets off of the main codebase without needing a central host. Much easier to maintain a private branch. Cons: more complicated model, less familiar, much newer system (I've never used it, just received a recommendation) 
Other systems can be found here: http://en.wikipedia.org/wiki/Comparison_of_revision_control_software