Possibly not much use to flightsimmers, but still an excellent and addictive site. Specify two locations (by IATA, ICAO or lat/long) and this will show the best great circle route between them. Somewhat easier than messing about with globes and bits of string.
Oh, one feature that may be of interest to simmers - you can enter your start point and the range of your aircraft in nm, and it will highlight the area of the world that you can fly to.
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