Geography Network has web-based mapping, data to download and access via ArcExplorer, as well as very useful “About GIS.” http://www.geographynetwork.com/
GeoCommunity is a source for data, tools, tutorials, and more. http://www.geocomm.com/
GIS Lounge has news, users forum, data links, material for teachers, and more. http://gislounge.com
GIS.com General information about GIS & spatial data. A good, basic introduction to the technology. http://www.gis.com/
The Alfred Wegener Institute Foundation for Polar and Marine Research Marine GIS Page has links to data and tools.
North American Cartographic Information Society. http://www.nacis.org/
Association of American Geographers. http://www.aag.org/
Geospatial Information and Technology Association. http://www.gita.org/