Interactive Google Maps based Great Circle Route mapper shows actual/ideal shipping and airline routes against a mercator projection. Southern California to Southern England example[1395x694]. Link inside. : r/MapPorn
RouteFinder (free access area)
AOPA Flight Planning Tools - AOPA
Mission Planner Flight PLAN — Mission Planner documentation
FlightConnections - All flights worldwide on a map!
What is Trip Planner and How Can I Use it?: AirTreks