Besides the NASA also the European Space Agency (ESA) provides access to a number of free datasets.
EOLi (Earth Observation Link) is the European Space Agency’s client for Earth Observation Catalogue and Ordering Services.
An overview of the available ESA datasets can be obtained via the EOLi Client (see above) or on this website.
The Sentinel Satellites are ESA’s latest contribution to global earth observation. The free, full and open data policy adopted for the Copernicus programme foresees access available to all users for the Sentinel data products.
The first of the Sentinels is a radar satellite. After a quick self registration the data can be accessed at: https://scihub.esa.int/