You can browse the sources on the web:
git://git.eclipse.org/gitroot/rap/org.eclipse.rap.gitgit://git.eclipse.org/gitroot/rap/org.eclipse.rap.tools.gitIf you are using Eclipse EGit you can add the repository by right-clicking into the Git Repositories View and then choosing Paste Repository Path or URI.
Alternatively you can run git clone [repository-path] with your command line tool.
Our git repositories are mirrored on github, fork us ;-).
See the Incubator page for the git repositories of all RAP Incubator components.
For more information on git at Eclipse read this introduction.