The previous codes may be too old now, here's a new one:
99c .com domain
SPECIAL OFFER! Get Your New .COM For $0.99*
Plus ICANN fee of $0.18(USD) per domain name per year for .COMs. Applies to the first year only of one new .COM registration per customer. Cannot be used in conjunction with any other offer, sale, discount or promotion. Customers may not use gift cards, PayPal, or AliPay to redeem this offer. Offer expires after 10,000 redemptions or at 11:59 PM (Mountain Time) on 2/28/2013 or at Go Daddy's discretion (whichever comes first). Go Daddy reserves the right to deny use of this offer and/or cancel domains purchased using this offer if the offer is abused or used fraudulently, as determined by Go Daddy in its sole discretion. Offer valid for residents of the U.S. and Canada only
Although it says ".. US + Canada only" it works OK in UK, don't know about other countries.