If you listen to Pepsi Pulse stations on iTunes Radio (about 12 songs, no skipping) an ad will pop up and offer you a free song credit on iTunes. You can only get one credit per day, but there is no limit on which song you can pick.
Part of Pepsi's "Real Big Summer" promotion.

I used the Pepsi Pulse Alternative station, but the Pop station probably works too.

Instructions:
Go to your music app.
On the radio tab, select the "Pepsi Pulse Alternative" station.
Play about 12 songs, no skips. You can take breaks as long as you reach the 12 song limit that day.
An ad will pop up instructing you to tap on it.
Once you tap, enter your iTunes password.
You will be credited. From there you can click the "Choose Your Song" button to go to the iTunes Store.