Geomapper module

Fallout 3
The Geomapper Module is a special item that can be received from Reilly after completing the Reilly's Rangers (quest) and taking the unmarked quest Geomapping with Reilly. The device tracks data about any Primary Locations (marked locations on your World Map) you find from your Pip-Boy 3000 and can be offloaded by Reilly at the Ranger Compound. She will pay 30 caps per location found. It will also count the locations you have already discovered before receiving it, and you don't have to carry it around - just get it out of your house locker when ready to visit Reilly again, and she'll pay for any new locations.

This item can be replaced by Reilly (only in pc version) if you drop or sell it.