With the release of Amazon Cloud Player for iOS, Amazon has started a promotion for $2 in free Amazon MP3 credit. Acquiring the credit is pretty simple. Just head over to the promotional page and look towards the center of the website. You should see a large yellow button. Click on the button and copy and paste this promo code (MP3S4ALL) into the box. $2 in Amazon MP3 credit will now be added to your account.
This credit will expire on June 15th.
Update (6/17/2012): This offer has been extended through June 18, 2012.