In the Classic days, if you wanted to use a window that wasn't frontmost, you could click anywhere on it to bring it to the front. Mac OS X introduced a feature called "click-through" where certain controls, like the "traffic-light" buttons in the window title, would work even for background windows. Applications can also make other controls, or even the entire window, work with click-through. To make things a little confusing, in older versions, click-through was off by default for Carbon apps, but on by default for Cocoa apps.
Some people (including myself) never got used to click-through; if you too wish to have more control over what happens when you click on a background window, Klicko may solve your problem. Klicko also has two other convenient functions: window maximizing and bringing all windows of some application to the front.
Klicko is a System Preferences panel written in Cocoa. It doesn't hack the system, other applications, inject code or do anything magic.
What is new in this release:
Now suggests applications known to be incompatible, several bug fixes.
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