DoublePane allows you to quickly assign a window to the left or right side of your display. You can also set a window to full screen or restore it to it's original arrangement.

It's like what a lot of apps have built into them for their own windows, but this is system wide. So you can have a Safari window split screen with a note taking application for research, or like what I'm doing: a blog post window on the left with the source website on the right. Managed with either a menubar icon or shortcut keyboard commands.

Plenty of apps out there that do this. However, DoublePane is free for July 4th.

Pick it up in the Mac App Store for $0.00 today.