Useful sites

Reviews can be very important when making a committment -- to a new camera, to a auto mechanic, to a vacation spot. There are many places now where people submit their reviews and you get to read all about it. Ebay is huge in this area, every thing is based on buyer and seller reviews and many people make it a practice never to buy from someone who has not garnered lots of great reviews.
I used cnet Reviews recently before deciding on my new digital camera. Lots of information here, but the user reviews are what finally tipped the balance for me.
Trip Advisor can be useful when deciding where to stay. User reviews, and photos, can be an enlighting addition to the official hotel site.
One of the best collaborative sites I have use is Car Talk. Maybe you listen to Click and Clack on NPR, but have you ever tried their web site? Full of valuable information, my favorite is the Find a Mechanic section. It is how we found our trusted mechanic and how we will find our next one if he retires before we do, as he's been threatening to do.