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 Lets you set default layouts
Using a feature called "snapshots" users can set up different default window layouts that they can switch to and from on the fly.
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 cost money to use.
Pro Window hints
An intuitive way to change window focus.
Pro Allows for tiling
Slate allows for customizing the config file in order to emulate tiling windows.
Con Hard to configure
There's no graphical user interface for configuring Slate, it has to be done from the command line interface or by editing a config file, making it somewhat difficult and time-consuming.