I don't understand the need for programs like this....Mac already has a great built-in screen shot tool that lets you take screen shots of a selected rectangular area, whole screen, or even individual windows..with handy keyboard shortcuts at that (Cmd+Shift+n, where n = 3, 4, or 4 + spacebar). Could it be any simpler?

Grab seems a bit more versatile, and it comes with the system. I'm just working off the screen shot though.

But how does it know that I shot a .tiff?

Great little app.