Discover Card offers a 5% cashback bonus on a "seasonally rotating" list of spending categories. For the 2015, these bonus categories are as follows:
Q1 2015 (Gas & Ground Transportation): Gas Stations, Trains, Buses, Taxis, Rental Cars
Q2 2015 (Food and Fun): Restaurants and Movies
Q3 2015 (Summer Spruce Up & More): Home Improvement Stores, Department Stores, and Amazon.com
Q4 2015 (Holiday Shopping): Online Shopping & Department Stores purchases
Note that "Q3 2015" means from July 1 - September 30, 2015. Also note that even if you have a Discover Card, the enrollment is not automatic. You have to login and activate (sign up for) this offer with Discover.