http://www.aboutairline.com/default_eng.htm
http://www.aerosite.net
http://www.cardatabase.net/modifiedairlinephotos/
http://www.avitop.com/default.asp
http://www.aviationtoday.com/index.html
airtransportbiz.free.fr/index.html
the last two are more to do with the business side of aviation, but they are still very useful

and loads more on http://www.topsitelists.com/bestsites/Airline/

hope that helps,
adam