There's probably a couple, but one I know of for sure is Fuel Infusion [www.fuelinfusion.com]. They started close to where I live, so I've looked into them a little more than the others. They've done some pretty extensive testing (not all of which is online yet though). The product is safe for all combustion engines. They use the same stuff in tractors, trucks, cars, atv's, motorcycles, etc...
Anyway, good luck.