What is a good website for cheap plane tickets from America to Europe?

I am planning a trip in September to go to London from Chicago and then take the eurorail around Europe for 2 weeks. I want to fly into London but i'm not sure which city i want to fly back to Chicago from. Which website is best?What is a good website for cheap plane tickets from America to Europe?
I find the best flights on Mobissimo.com and AirGorilla.com

You could also go to uk.yahoo.com or yahoo.de and do a search for travel agents in the U.K. or Germany. It is sometimes cheaper to purchase your ticket in Europe, even though you are starting your trip from the U.S.

Sounds like a great trip!

Enjoy-
kayak.com

bookit.comWhat is a good website for cheap plane tickets from America to Europe?
I work in the travel industry, and I always refer my clients and my friends to the website BookMe. You can compare prices on all the major travel websites and save lots of money. Good luck, and check out the link below.