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Oh my freaking god! This is infinitely useful! How many times have I tried to get someone to a portion of a page or annotate a page when I could have just used this!
Seriously, it's not often it happens, but I'll be telling someone, "This email fax site is great, but there's a sweet deal at the bottom of the 3rd page when going through the FAQ section that links to an offer they probably forgot all about. I've had 4 friends sign up with that offer and they are all getting the discounted rates.", and then I'll spend about an hour more trying to explain exactly where to go when I could just go "Here's a pearltrees link."!
It is simply awesome. Forget the laundry list of links, just send a pearltree! And on top of that the UI is just incredible!