Where's the Source?
STP uses SVN, and is a multi-repository project, with one repository for each sub-project or component. Individual components can be built (in most cases) independently, but sometimes cross-dependencies exist, and of course a full roll-up build implies access to all the relevant repositories.
SVN repositories can be access through the HTTP protocol, for anonymous read-only checkouts, and the SVN+SSH protocol, for committer checkouts. Here are the repository locations.
There are other supporting repositories too that you may not be using on a day-to-day basis - we have a repository that contains the top-level build for all components and projects, a sandbox to serve as a temporary contribution space and an archive, which is where we put retired components, so that they don't clutter up the place.