I scanned my Stop & Shop card and got .30 cents off per gallon (I scored 200 bonus points at my last S&S trip). The list price was $3.49. But I got 16 gallons - so that's a savings of $4.80. Stop & Shop already has some of the cheapest gas around - so even if you don't have points - you'll probably do well there.
If you don't have a Stop & Shop gas station near you, a lot of Shell stations take Stop & Shop cards. You can find this out on their website.