If you'd like to get a feel for whether or not you're getting a good deal on a car you've been looking at, TrueCar has the dirt. TrueCar collects and analyzes data from 25% of the automotive sales in the US—a sizable portion, and more than enough to paint an accurate picture.
You plug in your ZIP code and search for the make and model you're thinking about purchasing. TrueCar returns information about the car, including dealer cost, factory invoice, average price paid in your area, and the sticker price. Based on those numbers and archival sales, the site charts out a bell curve, showing you where the sweet spot for wheeling and dealing is.
TrueCar also provides sales history for the previous six months which is handy to check and see if the sale price is falling. TrueCar is a free service. If you know of another service for checking the sales price of cars, share it in the comment belows.