Recs.
Updated
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.
Specs
Pros
Pro Extremely configurable
Slate is similar to Linux WMs like Xmonad in that it is nearly infinitely configurable. You can either use the default configuration file or just Javascript for very complex/dynamic settings. Slate also offers editing a ".js" or a ".slate" file for configuration.
Cons
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Development is no longer active on this project as of 2017.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.