I checked out Arrange, Cinch, Sizeup, Divvy, and Optimal Layout, and I like Arrange best of all.
It looks like Arrange can do everything that Cinch, Sizeup, and Divvy can, except for the nice arbitrarily-configurable shortcuts in Divvy. Arrange certainly has a good number of shortcuts for several predefined actions, but it may be nice to also be able to assign shortcuts to custom freeform window arrangements in Arrange.