There is now an official git-annex package for Arch, so to install it:

pacman -S git-annex

There are at least three non non-official packages for git-annex in the Arch Linux User Repository. Any of these may be installed manually per AUR guidelines or using a wrapper such as yaourt shown below.

  1. A git-annex package is available in the haskell-core AUR

  2. A development package is available at git-annex-git that functions similarly to the source package but builds directly from the HEAD of the git repository rather that the last official release.

    $ yaourt -Sy git-annex-git
  3. A Cabal sandbox build is also available

    $ yaourt -Sy git-annex-cabal

Finally you may choose to forgo the Arch Linux package system entirely and install git-annex directly through cabal.

$ pacman -S git rsync curl wget gnupg openssh cabal-install
$ cabal update
$ cabal install git-annex --bindir=$HOME/bin