Right now through 4/1 you can get a FREE $2 Amazon MP3 Credit! In order to receive it, you'll need to enter the promo code HITUMBLR to add the $2 credit to your account. Your credit must be redeemed by 11:59 p.m. PST on 4/1. Limit one promotional credit per customer.
With Amazon MP3 you can buy a single MP3 starting at $0.99 or use this credit to get a discount on a complete album. Amazon has 100 MP3 albums for only $5 in March.