I am not sure if these (or similar ones) have been posted before but I thought I would share.

If you are trying to figure out what backspacing a wheel has and all you know is the offset, here is a calculator to determine the backspacing of a wheel.
http://ww w.autolounge.net/calculators/tirespacing.html (fix the www to make the link work)

If you are trying to figure out how much your new wheels will sick out (or in) from your current wheels, try this site.
http://ww w.1010tires.com/WheelOffsetCalculator.asp (fix the www to make the link work)