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Aggregate Knowledge isn't necessarily a name you're familiar with, but that's because they're not a consumer-facing service. Instead they power product recommendations for sites like Overstock.com. If you've seen Amazon's "People who viewed this item also viewed...", then you'll know how this works.
What's great about Aggregate Knowledge is that they can implement this system on any site, and the recommendations are updated in real time (Amazon takes a month to crunch the data). In addition to products, Aggregate Knowledge works on news sites, too - on product sites, AK gets paid for increasing sales, while on news sites it's all about increasing pageviews.