Vdot Calculator and Race Paces - used by Team P.O.D.
McMillan Running Calculator - a must for Team P.O.D. to really see how much pain you should be in at different distances.
Runworks Multi-function Calculator - recommended by Tom Weitzel. He says "In addition to normal stuff it calculates equivalant race times for different altitudes, elevation changes, temperatures, inclines or wind speeds."
Age Graded Results Calculator - to see if you actually beat the youngster ahead of you on an age-graded scale (or to see if you're maintaining or improving your fitness as you get older).
Team Oregon Pace Wizard - can be used to compute effort based paces and pulse rates for training and racing.