For 3D games, you can buy a gamepad. But for 2D games, a keyboard is sufficient.
Note: Controls below are even perfect for Mortal Kombat, Pokemon, etc. And if you are left-handed: the LR, X/Y, and A/B controls may be interchanged!
I might show the best Player 2 controls soon. If I have time - maybe the best Project 64, ZSNES, and ePSXe keyboard controls! ;D