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.
Provides imagery from the WorldView, QuickBird and GeoEye sensors.
Provides imagery from SPOT and Pleiades sensors.
Provides UAS solutions for purchase.