I like indo.com/distance, but keep in mind that it's approximate. It seems to do a city-to-city calculation, whereas airlines use airport-to-airport figures. For instance, it gives one figure for Boston to London, but AA (perhaps others; it's the only carrier I've taken from here to both) gives different mileage credit for flying into Heathrow and Gatwick.

Use it for planning, but if it tells you that you'll have 79,996 or 80,005 miles toward that 80,000-mile award, call your airline.