Filling up on gas at an Exxon station can now earn you a discount on a T-shirt at Macy's: That's the idea behind Plenti, a loyalty program launched earlier this year that lets users earn rewards for purchases from a variety of businesses. I tried Plenti for a few months, and although it takes a few minutes to set up, I found it to be an easy way to earn a few bucks back on money I was spending anyway.