If you have an interest in navigation in its broadest terms - from aircraft Cat III landings though sailing for pleasure to orientation strategies with young children, then you should consider membership of the RIN. http://www.rin.org.uk/
JavaScipt calculator for pilots. Determines density altitude from the altitude, air temp, altimeter setting and relative humidity. http://wahiduddin.net/calc/calc_da.htm
Publisher of flight information, supported by computer flight planning services, aviation weather services, and aviation training systems. http://www.jeppesen.com/wlcs/index.jsp