Programmer wanted...

[waldronate]

FT will do what you're asking, I think.

1) Load the map as an image overlay (equirectangular only, but ReprojectImage at http://www.ridgenet.net/~jslayton/ReprojectImage.zip will convert the projections)
2) Zoom and pan to get the desired area(s).
3) Use the ruler tool to measure distances (it does great circle distances).

If you don't have a retail copy of FT then you can download the demo from ProFantasy. It won't export things at high resolution or save things past the initial demo period but it will still measure things even after it's expired.

[waldronate]

As an aside, I like the Aviation Formulary for general great-circle computations. http://williams.best.vwh.net/avform.htm gives the forumulae that you'll want ot know for general navigation on a spherical earth. It's quite similar to navigation on an ellipsoidal earth, but the 1 part in 300 deviation isn't usually that important unless you're trying to enter a harbor or not fly into a mountain or suchlike.