Most gas stations offer some sort of credit card with a rewards program. Many of these programs allow you to earn points based on how many gallons of gas you purchase. Some gas stations like Speedway and SuperAmerica offer a rewards card that is not a credit card, instead you present the card before each fuel purchase to earn points for fuel and other rewards.
Many major credit card companies offer gas rewards as well. PumpAndSave.com has a list of the best gas credit cards. Also, contact your current credit card company to see if they can offer you gas rewards on your card.
Read more for a list of credit cards from major gas companies.