What open databases with aircraft, airlines and country exist?
Geographic (routes, land-based infrastructure) data:


http://ourairports.com/data/
http://openflights.org/data.html


Coordination (scheduling, flight numbers) data:


http://www.virtualradarserver.co.uk/FlightRoutes.aspx


Aircraft (registration) data:


List of national government registers - http://www.gatwickaviationsociety.org.uk/registers_official.asp
ICAO address country prefixes - http://www.kloth.net/radio/icao24alloc.php
(old) id and routing data - http://www.libhomeradar.org/download/index.php