NASA's Earthdata Search (formerly Reverb) allows you to search, discover, visualize, refine, and access NASA Earth Observation data.
The Data Pool is the publicly available portion of the Land Processes Distributed Active Archive Center online holdings.It offers ASTER data covering the US and its territories and global MODIS data. It offers FTP access to selected data products. All data are available at no cost.
The GDEx data access interface is the result of collaboration between the Land Processes Distributed Active Archive Center and George Mason University's Center for Spatial Information Science and Systems.
GloVis is an online search and order tool for selected satellite and aerial data. Through a graphic map display, the user can select any area of interest and immediately view all available images within the USGS inventory for the specified location.
EarthExplorer provides online search, browse display, metadata export, and data download for earth science data from the archives of the USGS.
The Government of Canada Geospatial Data Extraction Tool (formerly GeoGratis) provides free access to geospatial data and maps published by Natural Resources Canada. Their Geospatial Data Extraction tool allows for selected vector and raster datasets to be downloaded at custom extents.