Debian testing or unstable

sudo apt-get install git-annex

Debian 7.0 "wheezy":

sudo apt-get install git-annex

Note: This version does not include support for the assistant. A backport is available with the assistant and other new features.

Follow the instructions to enable backports.

sudo apt-get -t wheezy-backports install git-annex

Debian 6.0 "squeeze"

Follow the instructions to enable backports.

sudo apt-get -t squeeze-backports install git-annex