The Super Nintendo Entertainment System lock out is completely cosmetic. If you compare a SFC cartridge with a SNES cartridge, you will notice that the SNES cartridge has two notches in the back that the SFC cartridge does not. Now, push in the flap where you plug in the cartridge on your SNES deck. If you look inside, you should see two plastic pieces sticking up that correspond to the notches on your SNES cartridge.

You now have two choices. You could cut notches in your SFC cartridge to match the plastic pieces inside. Or, you could simply cut the plastic pieces off. I recommend the second choice; it is easier and less noticeable. I have found that the easiest way to remove them is to take a pair of hand-held garden shears to clip them out.

The same tabs exist on the SNES Game Genie and can be removed as well. If you are uncomfortable with either of these methods, you can buy adaptors which essentially extends the cartridge slot. The only adaptor I would consider buying would be the Super 8 adaptor. It allows you to play SNES, SFC, NES, and Famicom cartridges on your SNES deck. Unfortunately, I have never actually seen anyone selling this. Mark Drefahl has also suggested that the X-band device for the SNES also makes a good adaptor without modification (ed. He has no SFC cartridges to test it with, and I have never seen the X-band device. Not sure if actually works or not).