WWW.Consumerreports.org. A subscription is reasonably cheap and what you get is fantastic information.
WWW.Edmunds.com. Tons of information and pricing on just about every vehicle in the world.
WWW.KBB.com Pricing for both retail and trade values, on any used vehicle.
WWW.leaseguide.com Current lease pricing on almost any vehicle. Helps with the lease or buy question
WWW.carfax.com Find out if the used vehicle you are looking at has been in a serious accident. Very accurate and some dealers offer this information for free.
There are other sites that can help in your shopping or pre-shopping work. In the list above are sites recognized and respected by the dealers, as well as the public.