Gas prices are high enough as it is so it's always nice to get a little bonus for putting up with such exorbitant costs. Forum user panda470 has found that you can earn up to three Aeroplan miles for every $3 you spend at Esso gas stations. Full details can be found here, but the gist of the text reads:

• Earn 1 Aeroplan Mile for every $3 dollars spent on purchases of Esso Regular Grade, car washes or convenience store items
• Earn 2 Aeroplan Miles for every $3 dollars spent on Esso Extra Grade
• Earn 3 Aeroplan Miles for every $3 dollars spent on Esso Supreme Grade

Obviously, you'll have to be an Aeroplan member to take advantage of the promotion.