David Rumsey Map Collection
Thousands of historic and rare maps in high resolution

Old Maps Online
Search or browse across multiple historical collections by geography and date

Alexandria Digital Library, from UC
Includes link to international as well as U.S. data sources.

Perry-CastaƱeda Library Map Collection
University of Texas at Austin's large collection of digital maps from around the world

Ancient World Mapping Center
Cartography and GIS within the field of ancient studies

Introduction to GIS
Includes how to get the software, manuals, tutorials; from Amherst IT.

Starting the Hunt: Guide to Mostly On-Line and Mostly Free U.S. Geospatial and Attribute Data 2nd Edition

Infomine: Maps and GIS
Searchable, with links to thousands of resources

U.S. Geospatial Datasets: Federal Government Agencies (via CU)

National Map (USGS)
Public access to high quality geospatial data from consortium of federal, state, and local partner

Massachusetts Geographic Information System

Atlas of the Biosphere
Downloadable data/maps about the environment, including land use, human impacts, ecosystems, and water resources.

National Geologic Map Database
Print and digital U.S. geologic maps, many with downloadable data.

EROS (Earth Resource Observation Systems), from USGS
Data management, systems development, and research field center for USGS National Mapping Division

CSISS: Center for Spatially Integrated Social Science