git-annex.app

For easy installation, use the prebuilt app bundle.

To run the git-annex assistant, just install the app, look for the icon, and start it up.

To use git-annex at the command line, you can add git-annex.app/Contents/MacOS to your PATH

Alternatively, from the command line you can run git-annex.app/Contents/MacOS/runshell, which makes your shell use all the programs bundled inside the app, including not just git-annex, but git, and several more. Handy if you don't otherwise have git installed.

autobuilds

An autobuild is also available, thanks to the Datalad project.

using Homebrew

git-annex is available in Homebrew.

using MacPorts

git-annex is not available in MacPorts, but can be built from source using MacPorts tools. See MacPorts.

building it yourself

See porting.