Many governments, independent organizations, and agencies have Satellite Data that is Open and Free for anyone to access, study, analyze, and experiment. There is too much data; one needs to focus and pick the ones suitable for specific purposes.
Bhuvan
India has been launching satellites since 1998. The Satellite Data is available on the Bhuvan Indian Geo-Platform. Most of the data is India-specific. The information includes IMS-1 (Hyperspectral), Cartosat, OceanSat, and ResourceSat.
Copernicus Open Access Hub
Copernicus Hub, earlier known as a “Sentinels Scientific Data Hub,” provides the newest images from all Sentinels: Sentinel-1 radar imagery, optical multispectral imagery from Sentinel-2 as well as Sentinel-3 land products for environmental analysis together with atmosphere and air quality data in Sentinel-5P.
DigitalGlobe
Though not all are free, DigitalGlobe gives you a good amount of 30-cm high-resolution satellite imagery. You can use their DigitalGlobe Open Data Program to access these imageries for relief — fires, floods, hurricanes, typhoons, and earthquakes.
EOS LandViewer
LandViewer is part of EOS (Earth Observing System), one of the official distributors for high-resolution satellite images. LandViewer has CBERS-4, Sentinel-1, 2, MODIS aerial NAIP, Landsat-7, 8, and 4, 5 for historical photos. Among the datasets are SPOT 5-7, Pleiades-1, Kompsat-2, 3, 3A, SuperView-1. Some of the best spatial resolutions go as far as 40-cm per pixel.
NASA’s Earth Data Search
Most of NASA’s Earth Data is an analysis product for the study of Earth Science – such as the biosphere, cryosphere, hydrosphere, or atmosphere.
NASA Earthdata has data on global land use and cover – specialized satellite-derived data like permafrost and wetland types.
NOAA: Data Access Viewer
NOAA’s Data Access Viewer has datasets mainly along the coasts. It holds satellite, aerial, and LiDAR imagery.
Sentinel-Hub
EO Browser has multiple medium and low-res images. These include the complete collections from all Sentinel missions, Landsat 5, 6, 7, and 8, Envisat, Meris, MODIS, GIBS, and Proba-V.
Sentinel Playground can be used for satellite imagery mosaic of Earth gained from Sentinel-2, Landsat 8, DEM, and MODIS.
Download the ESA’s open-source Sentinel-2 Toolbox. The toolboxes give you the framework to a rich set of analysis, visualization, and processing tools.
UNAVCO
University NAVSTAR Consortium, UNAVCO, provides satellite imagery for scientific research. UNAVCO provides access to geodesy data such as flooding, plate tectonics, and earthquakes. Their platform — UNAVCO SAR Archive Search provides Synthetic Aperture Radar (SAR) Data.
USGS Earth Explorer
USGS has one of the oldest collections of various free GIS data, accessible through their EarthExplorer. It offers 50 years’ worth of comprehensive satellite imagery. All gathered from USGS-NASA and various NASA remote sensors. This is Terra and Aqua MODIS, ASTER, VIIRS, and many others. There are open datasets produced under collaboration with ISRO and ESA.