##Amethyst (open source, free)
Amethyst (open source, free)
Tiling window manager for OS X similar to xmonad. 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.
Source code: http://github.com/ianyh/Amethyst/