There's a new site within the series of tubes that's aimed towards helping people not get ripped off by their mechanic. RepairPal allows you to enter the year, make and model of your car, and then select what repair you need to have done (within certain limits). It will then give you a price range, which is based on a number of variables (including allegedly "top secret" price quote data used by shops).
A couple of early limitations I found with the site were a lack of data for older vehicles (neither my 1981 RX-7, nor my 1981 Oldsmobile 98 Regency were listed in the site's database), and a lack of data for all potentially necessary repairs (e.g. there was no data for the replacement of a purge control solenoid for a 2005 Mazda 3, a repair that's quite common).
The benefits to the site more than likely outweigh the above-noted detriments, however. You're able to save data about the repairs you've had done to your car, and export the data as widgets that can be embedded into Craigslist or eBay postings. Handy stuff! As more users enter real-world data, the site will continue to get better as well. Which is why it's important for the blogosphere to make consumers aware of such a site, so it gets the use it deserves!