Sonic Ultra 1: The Return (PC)
At the Sound Test screen, type 'cheater'. A button should appear. Click it to go to Level Select. To play the final bosses, choose Enter a Password from the Game menu. Type fb1, fb2 or fb3 for final boss 1, 2 or 3 respectively. Click OK. Make sure that you're in a level when you do this, otherwise you could end up playing as both characters at the same time.