Horizon Systems' NHD Plus

From the site: "NHDPlus is a geo-spatial, hydrologic framework dataset envisioned by the US Environmental Protection Agency. The EPA Office of Water, assisted by the US Geological Survey, has supported the development of NHDPlus to enhance the EPA Watershed Assessment, Tracking & Environmental Results (WATERS) activities and the USGS National Water Quality Assessment (NAWQA) Program's SPARROW modeling. Since its first release in 2006, NHDPlus has been made available by EPA to the wider water resources community and has been used for many diverse applications inside and outside of EPA and USGS."

The site includes data downloads regarding the geospatial hydrologic framework of the United States.