Here are links to some of my favorite websites :

photodatabse sites :

http://www.jetphotos.net
http://www.airliners.net
http://www.flugzeugbilder.de

Discussion forums

http://www.acig.org
http://www.flugzeugforum.de
http://forum.keypublishing.co.uk/
TankNet

Spotting information

DUS-Forum
Scramble
Spottinglinks