OverlayIt is a plug-in that lets the developer create " Mac OS X overlay windows ". Such windows are at the top of the application, don’t receive any event (clicking in the window sends the click to the object that is behind the overlay window), and are transparents. Once you have created an overlay window with the plug-in, you can draw a message in it.

Here is a screen shot of 2 overlay windows:

The green window displays the time, the red one lists the running processes and is updated every second.