Different sensors
Sub 2.5m resolution http://www.satimagingcorp.com/satellite-sensors/
2.5-20m resolution http://www.satimagingcorp.com/satellite-sensors/other-satellite-sensors/
An exhaustive list https://www.itc.nl/research/research-facilities/labs-resources/satellite-sensor- database/
Important ones to remember Geoeye, worldview, ikonos (purchase) Spot (purchase) Landsat (free, earthexplorer) Aster (free, earthexplorer) AVHRR (free) Hyperion (hyperspectral) MODIS (free, I think….) Radarsat (radar data, purchase) SRTM (http://www2.jpl.nasa.gov/srtm/cbanddataproducts.html. Drill in using link below 5. free. Earthexplorer)