There's also the I Travel, You Travel Calculator:

http://www.ityt.com/mileagecalculator/index.php

And the Webflyer Mileage Calculator:

http://www.webflyer.com/travel/milemarker/

But, vtqanh is right, Flyertalker rmyadsk's Flying Fish is a great tool:

http://flyingfish.oscarfish.com/?zone=download - You can view the discussion located here.