Mac only: Accurately named menu bar app Cinch lets you set windows up side by side, à la Windows 7's Aero Snap feature, just by dragging them to the side of your screen.
After installing Cinch, you are given the option of having it launch at login, and whether you want to see it in the menu bar. The app itself is very simple — to snap a window to half-screen size, just drag it until your cursor hits the side of the screen, and a dotted line will show you where the window will go. Releasing the mouse button will snap it into place, allowing you to have two windows side by side to work with. In addition, you can drag a window to the top of your screen to maximise it. After you're done working, drag the window away from its screen edge and its original size will be restored.
Cinch is free as nagware, costs $US7 for the nag-free version, and is Mac only. Thanks, Jasmeet!