Cycles the default audio endpoint for Windows when launched.
If you have multiple audio devices, this program will change the default to the next enabled device in the list. I use it to switch between my speakers (connected to an analog output) and my headphones (connected to the optical audio output).
The advantage over using Windows UI: you don't need to open the Audio control panel, select the new device, and activate it each time you want to change. Just run the app. Since there are no UI popups, it doesn't interfere with fullscreen games or video playback. I've got the program bound to a macro key on my keyboard--the switching is pretty instantaneous.