I’ve already showed you how to use Wii controllers on Android, so why not Xbox 360 controllers as well? And by “Android”, I mean 3.2+ versions where it isn’t already made to work from the manufacturers, which I know is the case with some tablets. Basically what you need to get this working is two things: a USB wireless adapter made for connection wireless Xbox 360 controllers to PCs, and an app called USB/BT Joystick Center. You connect the USB adapter over USB (hoping that the USB port can provide enough power), open USB/BT Joystick Center, give it permission to access USB and basically just follow the instructions. If reading the included help section is too much of a hassle, you essentially just go to the input settings of your device, enable the input method created by the app, and select it in your keyboard settings. Back in the app you let it find your controller and load drivers automatically.
Then it’s time to play games. On a lot of emulators you can configure input yourself, so you can just use the standard keymappings. On games like GTA 3, ShadowGun etc (yes, those two and more are supported) you need to map buttons specifically for the game from within the app. Keymappings for some popular games can be found on the developer’s website. Essentially the app makes the device think that the controller is a keyboard, and that e.g. X on the controller is X on a keyboard (or backspace for that matter).
USB/BT Joystick Center also has a very new analog stick feature that allows you to get full analog control over supported apps. Right now there’s essentially one, Mupen64Plus, and it kept force closing on my Galaxy Tab so I have to wait for the N64oid update to bring support (it will) to try it out myself. That means that for other games/emulators, the analog stick is essentially a 4-way D-pad for now. Still, better than a touchscreen.
If you already have an Xbox 360 controller, you need the wireless adapter. Microsoft sells an official one, but I cheaped out and went for this $10 version, and it works perfectly.