Interactive Guide
Wheel/Tire fitment guide - compare settings like wheel size, tire size and clearance.
PS: Set the left side to stock size, and right side to play around with aftermarket options.
Wheel Offset Calculator - calculate wheel offset
Tire Size Calculator - see what happens to your speedometer reading when you change the size of your tires and wheels.
Tire Size Calculator 2- compare different size tires
Tire Size Calculator 3 - This service calculates various aspects of a tire given its size specification.
Drop guide - shows how your car will look lower or lift.
Tire Dimensions Made Simple - compare different size tires (Thanks Gorilla Eg!)
fairly in-depth and very technical spread sheet which allows you to punch in wheel size, wheel weight, tire size, and tire weight and then spits out the moment of inertia of it. http://se30.dyndns.org:8080/SVX/rot_inertia_wheels.xls (thanks Mafdark)
P.S. -Feel free to ask me any questions if you have trouble reading the output or figuring out how it can help you choose your next wheel/tire.
If a link is dead let me know and I'll find another one.
If you have any other good sites post it, and I will add it to this post.