Wesnoth UMC Dev
There is a SourceForge project that supports a shared repository for people who want to collaborate on Wesnoth UMC (User Made Content) development. You can find it at its official website and its SourceForge.net page. Coordinated Wesnoth UMC Development Project.
You will need to have an SVN client installed on your development machine. Also, you must have an account registered at SourceForge in order to write files into the repository. Account registration is free, see the SourceForge account registration page for details.
In order to gain write access to the project, though, you will need to apply for membership to the project managers.