BetterTouchTool

This is a paid application to enable customizing the behavior of all your Apple input devices (remotes, keyboards, mice, touchpads, etc.) It's definitely worth the money for a lifetime license. Older versions that don't support current MacOS are also available for free from the software publisher that purchased BetterTouchTool from its original creator.

It adds some important capabilities:

• Remapping the behavior of the green “zoom” button, so it maximizes the window, rather than switching to Apple's full screen mode.
• Mapping three-finger taps and clicks to middle click, which is mega useful for opening webapges in new tabs.
• Window snapping, which behaves like the Aero Snap feature built in to Windows since Windows 7.

If you don't care about the input device stuff, the window snapping feature is available as a free app from the publisher.