OpenStreetMap is the only repository with a license suitable for inclusion with Maemo Mapper. Hopefully, someday, its coverage will be more complete.

Unfortunately, most of the commercial map sets (like Google's or Virtual Earth's) are covered by restrictive licenses and thus cannot be included with Maemo Mapper by default. It is questionable, in fact, that they can be used legally with Maemo Mapper at all. But, if you're willing to try them out (at your own risk, mind you), you can use the "Download..." button in the "Manage Repositories" dialog (accessible from the "Maps" menu) to download a set of additional repositories.

Any GPS receiver with bluetooth support of the NMEA protocol (via the Bluetooth Serial Profile) should work with Maemo Mapper. See the "Maemo Mapper : GPS devices that work... or not..." thread for some examples.