Flexiglass is a Mac application that might come in handy when working with multiple windows of other running programs. This simple yet efficient utility helps you immediately resize and arrange any opened window on your screen and comes with easy to configure settings.
You can work with this app whenever you wish to compare two or more documents on your desktop. All you need to do is select the proper window option from the main menu of Flexiglass (you can resize and move the window which the mouse cursor is currently over) and the program will immediately apply the change.
Another way to resize and move windows on your screen is to drag them to the margins of your monitor or hit a certain combination of keys. You can change the shortcut keys by going to Preferences -> Layouts menu and enable other useful features (Quick Layouts, Double click to Zoom, etc.).
The application also provides you with various instructions within the Preferences menu which teach you how to configure Flexiglass accordingly.
All in all, I'd say it's worth buying this window manager for your Mac. It's very simple to handle, uses a minimum amount of CPU resources, and comes with a very reasonable price.