Macs have great gesture-based window management, but keyboard shortcuts are largely absent. Spectacle corrects this oversight by allowing you to move and resize windows with simple keystrokes.
Spectacle is nearly identical to previously-mentioned SizeUp, but completely free. Mappable keyboard shortcuts allow you to snap windows to any side or corner of the screen, shift them to a secondary monitor, or launch them into fullscreen mode without touching a mouse. A history feature even lets you undo and redo the last several window actions. Spectacle is free and available in the Mac App Store.