Saucy

sudo apt-get install git-annex

Raring

sudo apt-get install git-annex

Note: This version does not have the WebApp enabled (see here), but is otherwise usable.

Precise

sudo apt-get install git-annex

Note: This version is too old to include the assistant or its WebApp, but is otherwise usable.

Precise PPA

https://launchpad.net/~fmarier/+archive/ppa

A newer version of git-annex, including the assistant and WebApp. (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.