Sta-Bil seems to be the most popular brand out there, and I've used it in my tank with no ill effects. I've heard good things about the Pri-G stabilizer/gas treatment stuff too. It's more expensive, but sounds like it does a bit better than the mainstream.

Basically, you are trying to keep the solvent elements from reacting, keep water out of the system, and keep the whole mess from polymerizing.

Does it make anyone else wonder why these chemicals aren't already added to gasoline by the producers to keep gas from going bad so fast? We add all sorts of chemicals to our food to keep it from going bad, and it doesn't seem to raise any economical issues there, so why not do the same with our fuel? I mean, how much do you suppose it is costing every year for people to have to remedy problems caused by bad/old fuel? Burning cruddy gasoline most certainly creates more pollution, so if for no other reason, mitigating the environmental impacts alone would seem to be worth the effort and cost in this day and age.

The more I think about it, the less sense it makes, except that someone's pockets are getting lined deeper and deeper with my hard earned cash for no other reason than greed.