There are already a couple free sites out there. They use reviews from customers (unfortunately due to the way it is done, they are mostly bad reviews, but not all). As we have discussed in other places, some of these are subject to "loading" either by the company or a truly disgruntled customer, but they give you a starting point. The two I have run across (not particularly fond of either btw) are Service Magic and Angel-something or other (can't find it now to link but it may be a local service for Pittsburgh, PA). Service Magic is heavily advertised and very intrusive. They tend to send spam to you about potential upcoming projects and vendors they push for them, even if you don't say you have anything. I can tell you, though, that there are A LOT of sites like what you propose, but none that really have taken off because it is so fractured, and with so many specialties, so many vendors, and so many people with different ideas of what makes a vendor good, that it may prove impossible. Just a few thoughts.
NOTE: During this writeup I have also found out they (Service Magic) are in litigation for false advertising. They state that they vet vendors before listing whereas they do not, they list any vendor willing to pay them according to the suit- so you may want to steer clear there. I don't know either way personally.
Second Note: I removed the link in case there are issues with Service Magic. If you want to go there, a simple Google search will bring them up.