(description from website follows. generally, a good starting point to search for GIS data sets, including those pertaining to weather, climate, and renewable energy) geodata.gov is a web-based portal for one-stop access to maps, data and other geospatial services that will simplify the ability of all levels of government and citizens to find geospatial data and learn more about geospatial projects underway. This is part of the Geospatial One-Stop initiative, one of the 24 OMB electronic-government initiatives that will enhance government efficiency. The geodata.gov portal will accelerate the development and implementation of the National Spatial Data Infrastructure (NSDI) and includes state, local and tribal governments along with the private sector and academia as participants.
Contains various map databases on topics including topography, biodiversity, water use, etc. Also contains links to other GIS data sources. A good starting point for searching for GIS data from the United States.