Is there any good free online tool to collect coordinate positions given, say a trajectory; or a polygon. For example, retrieve all coordinates between two points, every 500 meters.
You might try using the ArcGIS REST Geometry Service, which gets installed by default with ArcGIS Server.
The Densify method would allow you to create points along a great circle path based on some spacing. You would provide the first an last points, or at least 3 points in the case of a polygon.
For example this url returns points every 200 km. between Longitude 98E, Latitude 24N and Longitude 20E Latitude 55N (degrees). This url makes the same request but asks for an html formatted response instead.