Here are some useful links to websites that might be of interest to the radarspotter.
FlightAware Started in 2005, the first to offer free flight tracking for private and commercial aircraft. Its free PiAware software is one of the easiest solutions for tracking Mode-S flights using a low cost SDR and RPi combination. And it shows all available MLAT plots unlike others that block some MLAT flights.
Flightradar24.com Great site to track aircraft on a map, including previous 16 days, and excellent Aviation Database to look up aircraft by hex code, registration and airline.
PlanePlotter Low cost extension for all Mode-S receivers. Share data and plot the location of non-positional aircraft using multilateration (Mlat).
Gatwick Aviation Society Another great extension for Basestation. Convert Mode-S hex codes into an aircraft type and registration using the online form or download Active Display.