Raring
sudo apt-get install git-annex
Precise
sudo apt-get install git-annex
Note: This version is too old to include the assistant, but is otherwise usable.
Precise PPA
https://launchpad.net/~fmarier/+archive/ppa
A newer version of git-annex, including the assistant. (Maintained by François Marier)
sudo add-apt-repository ppa:fmarier/ppa
sudo apt-get update
sudo apt-get install git-annex
Oneiric
sudo apt-get install git-annex
Warning: The version of git-annex shipped in Ubuntu Oneiric had a bug that prevents upgrades from v1 git-annex repositories. If you need to upgrade such a repository, get a newer version of git-annex.