USGS EarthExplorer

earthexplorer.usgs.gov

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The US Geological Survey's search and download portal for satellite imagery such as Landsat, plus aerial photographs, elevation data and other Earth observation datasets.

Why it is useful

A free route to real satellite and aerial imagery, including historical coverage that lets you compare a place across decades. It requires a free account to download, and it is geared towards people who work with geospatial data.