Information on GIS Data Types:
Digital Raster Graphics (DRGs) - USGS
Digital Raster Graphics - GIS Data Depot
Digital Line Graphs (DLGs) - USGS
Digital Line Graphs - GIS Data Depot
Digital Orthophoto Quarter Quads - GIS Data Depot
Data Sources Class Link
Data Sources for GIS (The Geographer's Craft Project)
Federal Geographic Data Committee (FGDC)
National Spatial Data Infrastructure (NSDI)
GeoSpatial One-Stop (NSDI)
USGS Spatial Data Transfer Standard (SDTS) Home Page
More to come!!!