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.
This needs to be split into two parts:
Only ArcGIS can edit map composition (.mxd) -- symbols, colours, labels, legend, embedded tables, etc., etc. -- and geoprocessing tools (.tbx).
However the data can be edited by any program that understands the file formats inside the package, which can be any Esri supported filetype, most common are shapefile, file-gdb, and rasters like geotiff. As the creator of the map package you have control over the file formats included. You could for instance ensure everything is a shapefile which is editable by anything calling itself GIS.
At the receiving end, unpack the .mpk file using any standard de-archiver, might have to add a
.zip extension for it to be recognized, and explore the contents with the tools of choice. After the edits are done, re-zip, rename to .mpk and send back to source for review.
If the only changes are new features added/deleted from existing feature classes or raster values, the source office will see everything as soon as they open the map. New feature classes (or shapefiles or image files) will have be manually added to the map composition, but that's a straightforward process.
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.