Northwest is offering 500 bonus miles for shopping online through their Worldperks Mall and spending $150 or more between July 1 and September 15. Registration is required. See the details on the offer, because some of their participating merchants (eg hotel, car, flowers) are excluded from earning the bonus.
All your online purchases should — of course — receive some kind of reward, whether it’s Northwest miles or something else. The best one-stop sources for learning about the best bonuses are the extensive RewardsDB charts and the Webflyer tool.
It’s always worth checking out these resources before making an online purchase of any kind. (And it’s not just miles – you can earn cash rebates, too.)