git-annex allows managing files with git, without checking the file contents into git. While that may seem paradoxical, it is useful when dealing with files larger than git can currently easily handle, whether due to limitations in memory, time, or disk space.
git-annex is designed for git users who love the command line. For everyone else, the git-annex assistant turns git-annex into an easy to use folder synchroniser.
To get a feel for git-annex, see the walkthrough.