There is now an official git-annex package for Arch, so to install it:
pacman -S git-annex
There are at least four 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.
The simplest method is to use the git-annex-bin package based on the prebuilt Linux tarballs. This package includes many of the binary shims from the pre-built package. Although common Linux system utilities have been stripped in favor of normal dependencies, the pre-configured Haskell libraries included out of the box make this an easy install. The disadvantage is the resulting installation is a bit on the heavy side at nearly 100M.
$ yaourt -Sy git-annex-bin
A git-annex package is available in the haskell-core AUR https://wiki.archlinux.org/index.php/ArchHaskell
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
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