I'm looking for a tool that can open ArcGIS Desktop 10 map packages (.mpk files) and allow in a simplified way for a third-party to add geology cross sections and well points.
Any suggestions? It must be "free" and easy to use.
As @blah238 said, I am not aware of any free software that can read ArcGIS map packages. You could open them by unzipping but you will then find a *.mxd that only ArcGIS can read.
Consequently, seeking an alternative workflow/architecture will be required and @blah238's suggestion of investigating a transactional Web Feature Service seems like a good one.