If there's one good thing that Windows 7 users have built-in that Mac users don't, it's window snapping. In Windows 7 you can drag a window to the top of the screen to maximize it, or to the left or right side of the screen to make it fill half the display.
BetterSnapTool is window snapping for OS X on steroids. Whether you want to snap and expand to full screen, half screen (either horizontally or vertically), one-third of a screen, quarter screen, centered or even centered or maximized on the next monitor, BetterSnapTool has a gesture or keyboard shortcut to do it.
Options include the ability to define your own keyboard shortcuts, all 17 of them, turn on or off the various screen edge snapping gestures, as well as a whole host of settings for the preview window. If you're a fan of manually resizing or moving windows, BetterSnapTool includes a system of window resizing and movement using a modifier key and the movement of your mouse -- you don't even have to click. You can even make the OS X window traffic lights do set things with right or middle clicks, or define a task to a title bar double-click.
As the name might suggest, there are other apps that do similar things to BetterSnapTool. Cinch, for instance, gives you some of the screen edge gestures, but none that I've tried have had such a comprehensive and easy to use list of snap options. Even the optional menu bar icon is rather attractive and looks like a mini Battenberg cake.