There is a non-official source package for git-annex in
[AUR](https://aur.archlinux.org/packages.php?ID=44272).

You can then build it yourself or use a wrapper for AUR
such as yaourt:

<pre>
$ yaourt -Sy git-annex
</pre>

----

I'm told the AUR has some dependency problems currently.
If it doesn't work, you can just use cabal:

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