1. Different sensors

2. Sub 2.5m resolution

3. 2.5-20m resolution

4. An exhaustive list
database/

5. 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 ( Drill in using link below 5. free. Earthexplorer)