A very good and up to date one, is http://www.bird.ch/bharms
. It's on Airbus, Boeing, McDonnell Douglas, and L-1011.
Another convenient site can be found at http://www.intercpt.demon.co.uk/fleets.html
. Mind you, this one isn't quite that up to date. But it comes in handy when you have to start looking by registration number, i.e. when you don't know the aircraft type.
Enjoy your search,