GPS-enabled cell phones and mobile devices have opened up a new world of online mapping, in which you and the crowd can add data to websites!
Many libraries or other collections have taken the time to scan historic maps, which can be used in a GIS system once they are georeferenced--some of these collections even have pre-georeferenced maps!
The UM Libraries subscribe to some databases that include maps...
Charts are maps that depict water, navigation aids, etc.
Gazetteers are wonderful resources - they list place names (this can be particularly handy for historical research, since place names do change over time) and provide latitude and longitude coordinates. They also give information such as square mileage, length, and height for various geographical features.