EarthExplorer – Many sensors, including Landsat archive, aerial imagery (including NAIP), AVHRR, EO-1 ALI and Hyperion, ASTER, certain MODIS, some radar and LiDAR, and Sentinel - http://earthexplorer.usgs.gov/

Montana State Library – A broad range of GIS data for Montana, including NAIP aerial imagery - http://geoinfo.msl.mt.gov/

DigitalGlobe – WorldView and GeoEye data - https://www.digitalglobe.com/

GloVis – Mostly the same data as EarthExplorer, but with a different interface - http://glovis.usgs.gov/

MODIS Web – A gateway to various MODIS data providers - http://modis.gsfc.nasa.gov/

Planet – RapidEye and PlanetScope data - https://www.planet.com/

Sentinel Online – European Space Agency Sentinel data - https://sentinel.esa.int/web/sentinel/home