Vector Htmls:
As most of these are United States Government sites, traversing the links might bring you to a Government warning. If this occurs and you were honestly traversing links provided, then continue... This warning usually appears only on FTP sites. If you are required to submit a password and 'anonymous' does not work, then I suggest you traverse elsewhere...

• All Layers
  USGS : DLGs
  USGS EROS data center
  USGS National Atlas of the United States™ (Shape files)
  Census T I G E R®

• Hydrology and Weather
  USGS Water
  EPA Water
  National Hydrography Dataset (NHD)
  Hydrologic Units, unit codes, and unit names
  (NHD Data) FTP site
  National Oceanic and Atmospheric Administration
  National Weather Service
  National Geophysical Data Center
  National Climate Data Center
  WMO: World Meteorological Organization
  The National Operational Hydrologic Remote Sensing Center (NOHRSC), operating under the Office of Hydrology of the National Weather Service, creates and distributes a variety of products that use remotely sensed data in support of the hydrological services mission of the National Weather Service. The National Weather Service issues river and flood forecasts for the Nation and provides hydrometeorlogical data and products to support the Nation's water resource managers.

• Vegetation
  DOI: Department of the Interior
  USGS Biological Resources Division
  USDA Forest Service Data and Information Systems
  Databases at the National Agricultural Library
  Plants
  Global plants
  National Wetlands Inventory
  Other Related Sites

• Soils
  DOI: Department of the Interior
  USDA Home page
  USDA Services
  USDA Agencies
  USDA NRCS
  USDA NRCS Natural Resources Inventory
  NRCS Technical resources
  NRCS GIS
  National Soils Survey Center
  USDA World Soil Resources
  USDA World Soils

• Transportation
  Bureau of Transportation Statistics (BTS)
  National Transportation Library
  National Transportation Atlas

• Urban utilities:
  Urissa: Urban and Regional Information Systems Association
  USGS Home page

• Vectors
  The Federal Web Locator
  CDC: Centers for Disease Control and Prevention
  FEMA: Federal Emergency Management Agency
  DOI: Department of Energy
  FCC: Federal Communications Commission

• Data types - Standards
  USGS Standards
  DLG: Digital Line Graph
  DEM: Digital Elevation Model
  BIN and IMG: Imagery... many types with HDR or DDR headers
  
  Spatial Data Transfer Standard Encoding Program
  SDTS
  Spatial Data Transfer Standard Document
  Part 1: Logical Specifications [Draft] (PDF file)
  Part 2: Spatial Features [Draft] (PDF file)
  Part 3: ISO 8211 Encoding [Draft] (PDF file)
  Part 4: Topological Vector Profile [Draft] (PDF file)
  Part 5: SDTS Raster Profile and Extensions Draft Version 1.1, January 1998 (PDF file)