If you want to know what's the shortest connection from point A to B on a sphere (~ as our planet earth), which is likely the approximate route along which a plane will travel (so called "beeline" or "as the crow flies"), I can recommend the following tools:
1) Luftlinie.org
2) Google Maps Distance Calculator
3) Related Wikipedia articles:
Great-circle distance in English.
Orthodrome, in German, but has better illustrations than the English article.
1) Luftlinie.org
- + Very simple to use: Enter text for origin and destination, confirm.
- + Nice GUI:
- Uses full available screen space
- Shows linear/road/orthodromic lines next to each other for good visual comparison
- Shows distance
- + Results are saved as nice URLs, can easily be shared.
2) Google Maps Distance Calculator
- - Handling is a bit harder.
- - Layout lacks a little bit.
- + Add arbitrary origin/destination point simply by clicking on the map.
- + Allows multiple sequential beelines.
- + Further features.
3) Related Wikipedia articles:
Great-circle distance in English.
Orthodrome, in German, but has better illustrations than the English article.