I love Konix's Speedking. Digital two button version with autofire, originally sold for some Sega IIRC, but it works just fine on Amiga too. It has helped me with many games with its separate second button... like with Project-X, Roadkill etc where you should reach spacebar with single button joystick to select or fire special weapons, but now you can just press the second button without losing the touch in the game.
