PS2/USB AdapterIf you've got PS or PS2 controllers around I wholeheartedly recommend this, the Dual Shock controller is easily the best gamepad I've ever used, and it's got enough buttons for every emulated system short of the Neo Geo fighter set. The analog sticks let you have a good experience with emulated 64 games too.
The only shortcoming is that they do not work all that well with flying games intended for a real joystick, a single mission in TIE Fighter using one of those thumbsticks and my hands were screaming at me.