ESRI - Makers of ArcView, Arc/Info, ArcGIS and ArcExplorer. They offer a wide range of products and free resources related to GIS.  ArcExplorer may be downloaded at the free resources link from the home page. Many examples of Internet mapping are also available at the GIS Online link.
 

US Census Bureau's Factfinder -  An interactive mapper that allows mapping at a range of scales from national to local.  Data in all census categories is available for 1990 and 2000.
 

Geography Network - Links to a wide range of GIS resources online. Highlight is ArcExplorer Web which draws data from many websites and displays it here.
 

US National Atlas - Data on everything from agriculture to health to water quality may be mapped here with outstanding resolution and zooming capabilities.
 

Ulster County Information Services - Data including transportation, floodplains, wetlands, parcels, etc. for Ulster County, similar to spatial data available through some other counties such as Westchester.

Westchester County GIS - A wide range of data for Westchester County, plus presentations from the NY State GIS Conference.

New York State GIS Data Clearinghouse - Spatial data from throughout New York.  Highlight is 1-meter resolution orthophoto coverage.