Part of this is from the WorldWind Workshop, some of the data sources are from bookmarks I have collected. Several of the links below include data from several other sources.
Public Domain Sources
National Digital Orthophoto Programs
http://www.ndop.gov/
The National Digital Orthophoto Programs (NDOP) was chartered in 1993 as a consortium of Federal agencies with the purpose of developing and maintaining national orthoimagery coverage in the public domain by establishing partnerships with Federal, State, local, tribal, and private organizations.
The National Map
http://nationalmap.gov/
The National Map is a collaborative effort among the USGS and other Federal, State, and local partners to improve and deliver topographic information for the Nation. It has many uses ranging from recreation to scientific analysis to emergency response. The National Map is easily accessible for display on the Web, as products and services, and as downloadable data.
USGS Global Visualization Viewer
http://glovis.usgs.gov/
The USGS Global Visualization Viewer (GloVis) is a quick and easy online search and order tool for selected satellite data. The viewer allows user-friendly access to all available browse images from a number of Landsat data collections as well as ASTER, MODIS, and EO-1 data.
EarthExplorer
http://edcsns17.cr.usgs.gov/EarthExplorer/
With EarthExplorer you can search, download, and order U.S. Geological Survey products from the following datasets: Aerial Photographs, Advanced Very High Resolution Radiometer (AVHRR), Declassified Data, Digital Elevation, Digital Line Graphs, Digital Maps, Earth Observing 1 (EO-1), Global Land Survey, Landsat Archive, Landsat Decadal, and Landsat Science. You can also search and review products from the Radar, Spot, and USGS Commercial datasets.
The National Map Seamless Server
http://seamless.usgs.gov/index.php
The National Map Seamless Server is the ultimate location to explore and retrieve data. U.S. Geological Survey (USGS) and the EROS Data Center (EDC) are committed to providing access to geospatial data through The National Map.
GeoData
http://gos2.geodata.gov/wps/portal/gos
geodata.gov is a geographic information system (GIS) portal, also known as the Geospatial One-Stop (GOS), that serves as a public gateway for improving access to geospatial information and data under the Geospatial One-Stop E-Government initiative. Geospatial One-Stop is one of 24 E-Government initiatives sponsored by the Federal Office of Management and Budget (OMB) to enhance government efficiency and to improve citizen services.
NGA-Earth
https://www.geoint-online.net/community/ngaearth/default.aspx
NGA-Earth.org provides access to geospatial data over the World Wide Web. This service brings timely geospatial intelligence (GEOINT) to all users with an Internet connection. Via this access point, NGA has been able to bring current imagery to first responders, aiding in rescue and recovery efforts, while also providing unclassified views of the world in support of its traditional military and intelligence customers.
Global Change Master Directory
http://gcmd.nasa.gov/
Their goal is to enable users to locate and obtain access to Earth science data sets and services relevant to global change and Earth science research. The GCMD database holds more than 25,000 descriptions of Earth science data sets and services covering all aspects of Earth and environmental sciences.
NASA Earth Observations
http://neo.sci.gsfc.nasa.gov/Search.html
Their mission is to help you picture climate change and environmental changes happening on our home planet. Here you can search for and retrieve satellite images of Earth.
Group on Earth Observations
http://earthobservations.org/
The Group on Earth Observations is coordinating efforts to build a Global Earth Observation System of Systems, or GEOSS. GEO is a voluntary partnership of governments and international organizations. It provides a framework within which these partners can develop new projects and coordinate their strategies and investments. As of June 2009, GEO’s Members include 79 Governments and the European Commission. In addition, 56 intergovernmental, international, and regional organizations with a mandate in Earth observation or related issues have been recognized as Participating Organizations.
DATA.gov
http://www.data.gov/
The purpose of Data.gov is to increase public access to high value, machine readable datasets generated by the Executive Branch of the Federal Government.
Government Sources
NGA WARP
https://warp.nga.mil/
The NGA Unclassified National Informational Library (UNIL) is an archive of commercial imagery, domestic and non-domestic, available for discovery and dissemination. The Web-Based Archive and Retrieval Portal (WARP), is the interface which provides the on-line access and retrieval capability via the World Wide Web (WWW), NIPRNET, or the NGA Sensitive But Unclassified (SBU) Network.
Army Geospatial Center
http://www.agc.army.mil/