View the best flight offers, search for low fares, and sign up for email notifications on the latest specials for European travel on Flycheapo.com.
The site doesn’t have the prettiest interface although it’s a powerful search engine if you’re willing to take a moment to work your way around it. I find that using niche travel search engines (like EuRail for trains) can produce better results than generic ones such as AirFare.com.