Is there a web site or service that can estimate value of things? (details)

For no reason whatsoever, I started digging through my mounds of Hot Wheels cars that I’ve had lying around since I was a baby. For some reason or another, I remember a friend in very early grade school who said he collected them and noted that one or two of my die-casts were “valuable.” Is there some sort of free service online that can help me guesstimate the given value of the few cars I have that may hold value?