macappsthatrock:

($6.99) – Cinch gives you simple, mouse-driven window management by defining the left, right, and top edges of your screen as ‘hot zones’. Drag a window until the mouse cursor enters one of these zones then drop the window to have it cinch into place. Cinching to the left or right edges of the screen will resize the window to fill exactly half the screen, allowing you to easily compare two windows side-by-side (splitscreen). Cinching to the top edge of the screen will resize the window to fill the entire screen (fullscreen). Dragging a window away from its cinched position will restore the window to its original size.

SizeUp by the same developer is also pretty awesome but it is not on the mac app store yet.