I love recommendation engines. My 2009 purchase history was notable in that I *nearly* managed to do all of my Christmas shopping online and that a large number of sites got to "know me" extremely well over the year, becoming near friends in their accuracy of what I would surely like to purchase. My favorites:
Pandora's Music Genome Project causes me to discover and purchase more music than traditional radio or MTV videos ever could. I have 39 fairly well-refined stations, about half of which were born from the initial artists that I chose.
Etsy's "handpicked items" while not at all reflective of my past purchases or browsing habits, still send me down a path of discovery of new artists that could amuse me for hours and nearly always ends in a purchase.
Amazon's "Recommendations for You" is always my first stop for new books to purchase.
Last.fm makes me understand crazy facts about my own music collection, such as the fact that my most played song of all time is one that was introduced to me 3 months ago (too much "repeat" clearly) and that my music tastes have evolved considerably in the past few years (with some embarrasing legacy artists still in my most played).
iTunes's Genius Recommendations make me buy almost as much music as Pandora, mainly because of the convenience.
Bluefly's recommendations on designer handbags lead me effectively from one that is not quite right to the perfect bag that I am more likely to buy.
Facebook's network effects let me type in the name "Greg G" for example and in the top few hits I will see the Greg whose last name I forgot, but who I was looking for.
Mint's My Financial Summary and pie charts make me far more self-aware of little expenses (ATM fees) and large ones (vast fortunes spent at Whole Foods).
Bandsintown scans my Pandora tastes to find the best upcoming concerts in the area for me.
My6Sense I'm just beginning to experience, but I am impressed so far at how it filters content to determine what is most valuable to me.
Now I have sites that know me inside and out in so many domains: music, crafts, books, designer bags, friends, concerts, financial transactions, blogs. I'm still looking for a good recommendation site for restaurants (don't say Urban Spoon; it's too random and while I love Yelp to check out restaurants, it doesn't recommend well). So far the best substitute is Tyler Cowen's Ethnic Dining Guide, especially because we share the same favorite Korean restaurant (Seoul Gool Dae Gee (Honey Pig Restaurant)).
But most of all, I lament that I can't take my well-refined music preferences honed on Pandora over to Amazon to recommend cross-domain purchases. What additional factors could my eventual universal ID recognize that would make the connection between my recent purchases of the Horse Feathers collection on iTunes; Jeff in Venice, Death in Varanisi on Amazon; a handmade alligator skin wallet on Etsy; and a designer leather hobo on Bluefly?
I'm looking at you, MyType.