I just found an open source window manager called [slate](https://github.com/jigish/slate).. yes its better than **divvy** and better than **sizeup**.. and it has a key binding for switching focus between screens. It's default key bindings are shown [here](https://github.com/jigish/slate/blob/master/Slate/default.slate) (very customizable).. it defaults to  <kbd>⌘</kbd><kbd>→</kbd> and <kbd>⌘</kbd><kbd>←</kbd>

**note:** *Don't* listen to those who way slate is all complex blah blah.. it's every bit as simple as the two other solutions.. it's just those two are a bit more visual. that's all.