Steam has an "Add to library" feature for Windows executables that allows you to add other programs to your Steam library, and then make use of the Steam overlay. This even works for the Nintendo GameCube/Wii emulator Dolphin! As you can see in the image above, click the "Games" menu ...
Windows will connect the controller by default, but it may not be usable as a controller in all apps. Dolphin, the Wii emulator, supports using them as inputs, but we didn't have any on hand to test system-wide use. Mac is supported in the same way---only in Dolphin. System-w...