Cinch is one of my favorite applications even though I hardly notice it anymore. I am surprised that Apple hasn't incorporated this feature into their OS.
The premise of the app is a simple one, resize windows quickly so that I can look at more than one window at a time. It also lets you quickly take a window and drag it to the top of the screen which will make the window take up the full screen.
The app is free with a once in while nag to purchase the app. You can click "continue using" and keep using it for free indefinitely. I recently purchased the app to show my support for the developers, but used the free version for months.
Give it a try. The paid version is currently $7, but they do have a free trial version which gets the job done. http://www.irradiatedsoftware.com/cinch/