How do you visually identify aircraft from the ground? Are there any good resources that teach you how to identify jetliners from the ground? I'd love to see some great comparative photos of their silhouettes. Books or websites are both ok.

For example, here's a plane that was flying over yesterday en-route to KSFO. I'm guessing it's a 747 or A380, but I can't easily guess from this angle.
The canonical reference for aircraft recognition would be Jane's All the World's Aircraft.  

http://www.airliners.net is also a useful resource, but generally means working from photos - the line drawings in Jane's are often easier to use because they abstract away things like airline liveries and distill the aircraft down to its basic shapes.