This is also a well done site. Barret Jackson has an on-line value guide that will tell you the selling price of cars sold at their auctions since 1988. This is a very easy to use feature. I decided to do a little research on my 1989 Subaru DL, the newest car I own. I was amazed to see just how much the values varied from site to site.
Some sites list two values for cars.
Retail value is the price you would pay if you bought a car from a used car dealer. These numbers are usually inflated. Wholesale value is the amount a dealer would pay you if you were making a trade in on a car or if you just had a car that a dealer wanted. The first site I tried was the Kelley Blue Book . Kelley is probably one of the most accurate sites for common cars.