Amethyst is a tiling window manager similar to xmonad. It was originally written as an alternative to fjolnir's awesome xnomad but written in pure Objective-C. It's expanded to include some more features like Spaces support not reliant on fragile private APIs. Amethyst is free and open source under the MIT license.
Note: Amethyst must be given permissions to use the accessibility APIs under the Privacy tab of the Security & Privacy preferences pane. There is a comprehensive list of keyboard shortcuts that are available on the developer's webpage.