Patrick Smith once published an article about the longest conceivable flights, and uses the Great Circle Mapper as a tool...

Long haul madness. A list of the world's longest flightshttp://www.salon.com/tech/col/smith/2005/11/18/askthepilot162/

(You can read for free; just look for the 'skip this ad' or 'enter salon' link on the gateway page)