In preparation for Cataclysm, Blizzard has been emailing those who have canceled or expired accounts. They offer 7 free days of World of Warcraft by reactivating the account without having to add a credit card or payment method. It is certainly a great opportunity (and a marketing strategy) for those who have taken a breather and it may be a window to return to ancient Azeroth for the last time. The offer is only valid until December 1.
First of all, be careful with the emails that can reach you with this offer since it is very possible that hackers get to work. The mail seems to be coming from newsletter@email.blizzard.com and they will take us to this address: https://www.wow-europe.com/account/claim-promotion.html?promoId=SEVEN_DAYS_PROMOTION