XClickThrough

XClickThrough is an application that captures your mouse clicks. By default, OS X disables "click through" on application elements.

This means that if an application is in the background, you have to click it once to give it focus, and again to make it do something. This behavior is really annoying on multi-monitor setups, because you can see the thing you want to click on, but you have to double-click it to make something happen.