A wet stone. Knife sharpening is becoming a lost art. Granted it takes quite some time to put a good edge on a blade, but it's well worth the effort.
You have to start with a good knife. Those stainless steel knives, I just throw away.
It will take me about 2 hours to sharpen my knife, but once I'm done I can skin about 10 to 12 deer with it (hitting bone sometimes) and it's still razor sharp.