Slate is a free window manager similar, originally written to replace Divvy and SizeUp due to some limitations in how each work. It attempts to overcome them by providing more configuration options.
It may be a bit daunting to get configured, but once it is done, the benefit is huge. Here is a great writeup by a dev getting Slate setup.
Slate currently works on Mac OS X 10.6 and above.
Ranked in these QuestionsQuestion Ranking
Pro Extremely configurable
Pro Based on hotkeys
Hotkeys can be set to re-size and focus windows, as well as activate layout presets.
Pro It's free
Slate is completely free to download and use, which is a great option in a field of Mac WMs that often costs money to use.
Pro Allows for tiling
Slate allows for customizing the config file in order to emulate tiling windows.
Pro Lets you set default layouts
By using a feature called "snapshots", users can set up different default window layouts in Slate that they can switch to and from on the fly.
Pro Window hints
Slate offers window hints which are an intuitive way to change window focus.
Con Project is no longer in development
Development is no longer active on this project as of 2017.
Con Hard to configure
There's no graphical user interface for configuring Slate, meaning it has to be done from the command line interface or by editing a config file. This makes it somewhat difficult and time-consuming.
Con Missing features
Unable to perform all tasks of software it was meant to replace.
Can not move windows between 'Spaces' (virtual desktops).
Con Video fail
Demonstration video failed to highlight any of the advantages of the windowing system. Or at least, the advantages were so subtly intrinsic that no-one who didn't already use the system could appreciate them.