A free app for Android, by Soft Igloo.

BT Controller is an Android app that turns your device into a controller for any emulator that supports custom buttons. With this app, you can play console games anywhere and control emulators on your TV, Tablet, or Phone. It supports up to 4 players, and you can use either Bluetooth or Wi-Fi to connect your devices.

If you want to create your own controller, you can do so. But you can also opt to choose from thousands of user-made controllers.

The app supports emulators that use the Android OS. It is now required by Google to use Bluetooth, for scanning purposes, so the app will not work with physical consoles.