WindowFlow is an application switching utility with a few differences. The program replacing the built in application switching facility that appears when Option and Tab are pressed simultaneously, but rather than simply displaying a running program's name and icon, a preview screenshot is also displayed.
When multiple instances of the same program are running, this makes it much easier to distinguish between each one and to ensure that the correct one is selected, and the preview also helps to give an idea of where on screen the program window is positioned.
Keyboard shortcuts can be configured to hide individual programs and folder windows from the list, and you also have the option of zooming in for a closer view.
Reading about the program can make it seem like a minor addition to your Mac but, as if often the case, it is minor changes to an existing idea that make all the difference. Switching between programs made significantly easier and the seamless way WindowFlow integrates with OS X is impressive.