From 9to5Toys.com:

Today only, until 7 pm ET, Best Buy takes 20% off iTunes eGift Cards, as listed below. At up to $20 off and no shipping, each is tied with our mention from four weeks ago and at or near the lowest price we’ve ever seen. Sales tax is added where applicable. The gift cards will be emailed for redemption so those overseas with US iTunes accounts can take part.

iTunes credit can be used for Mac Software, iOS Software, Games, iBooks, music, movies or just about anything besides hardware.