STATS America is a data portal where visitors can get demographic, workforce, housing, and other data for every state, county, and metro area in the United States. The site is jointly sponsored by the US Commerce Department and Indiana University.
The Red Cross and FEMA jointly operate the National Shelter System, a comprehensive database of over 54,000 emergency shelters across the United States. When shelters are activated during an emergency, this system is used to monitor locations, capacities, and other shelter information. In addition to Red Cross and FEMA staff and volunteers, many state and local organizations use this information as well.
The Africa Map project, currently in beta, is an online map and data viewer created by Harvard University. The map combines data sets of numerous categories from a wide variety of sources, allowing thematic maps to be viewed and explored. The purposes of the project, according to the project web site:
Interact with the best available public data for Africa
See the whole of Africa yet also zoom in to particular places
Accumulate both contemporary and historical data supplied by researchers and make it permanently accessible online
Work collaboratively across disciplines and organizations with spatial information about Africa in an online environment
"Envisioning Development: What is Affordable Housing?" is an interactive Flash-based map of housing and income statistics on a neighborhood-by-neighborhood basis throughout the New York City area. Although I've seen similar statistical/analytical interactive maps with similar data plenty of times, the aesthetic design and information visualization "impact" of this map is almost breathtaking. This map is part of an online resource toolkit for urban planning and land use education based around NYC, offered by the Center for Urban Pedagogy.
The Library of the University of Arkansas has a very impressive collection of international and US geodata repository links - mostly geospatial data and attributes. I'll be poking around in here for hours, I think! A wide variety of topics and well-organized to boot.