If you experience an Mac OS X program to freeze and you cannot exit it by clicking quit or using a keyboard shortcut, you can always force it to quit using the Activity Monitor.
Neat little trick to force the preview images of running programs in Windows 7 taskbar or a so called Aero Peek, to show instantly with no delay at all.
Pretty simple and useful tutorial, where you will open a random photograph with smoke and convert it into something really not friendly for our environment.