It’s a free article from the December issue of Inside Flyer and includes a comparison chart showing which of the different online shopping portals offer the most miles for a variety of popular merchants.
Note that while the article and accompanying chart are very useful, they’re not 100% comprehensive. It’s still worthwhile keeping Smarterliving’s MileFinder and Ronald’s Cross Reference Guide in mind.
Also note that contrary to the implication in the article, Goldpoints can be redeemed for miles with a whole sleuth of carriers (in addition to redeeming for Radisson hotel stays), not just for miles with USAirways and Northwest. The key is signing up for the Goldpoints program via Radisson Hotels (then take your account number over to the Goldpoints home page and choose ‘create a new login’).