Hazards Data Distribution System (HDDS) by U.S. Geological Survey
Provides archival imagery from LandSat, WorldView and many other sensors. Special requests can be made to USGS if needed with a majority of imagery provided as free downloads.
Google Earth Pro
Now free to use, Google Earth Pro allows users to download archival imagery as seen in Google Earth. However, users must manually geo-reference all imagery downloaded. When installing Google Earth Pro, follow the instructions to receive the free license key.
Provides imagery from the WorldView, QuickBird and GeoEye sensors.
Provides imagery from SPOT and Pleiades sensors.
Provides UAS solutions for purchase.