Called "Capitaine Train", it's based in Paris, and is a one-stop shop for buying tickets from some of the biggest passenger rail systems in Europe. As of now, they sell tickets on Eurostar (the "Chunnel" trains between London, Paris, and Brussels), SNCF (French National Railroads), DB (Deutsche Bahn - German Federal Railroads), three other lesser known systems (Thalys, Lyria, and Thello), and even a few bus lines.
It's easy to use. You don't have to get accustomed to the idiosyncrasies of each railroad's proprietary website, yet you still have access to all of their lowest rail fares. What's more, there are no booking fees (they earn commissions from the sale of the tickets), and they have real, live people at the other end of the email pipeline when questions arise.
My brother used it to buy tickets for travel in France and thought it was great. Testimonials by others echo that. You can create a unique account for use on Capitaine Train, or you can simply sign in using your Facebook credentials.
Heading to Europe and planning to use the trains? Click on over to Capitaine Train. (Select the U.K. flag to get the English version - the default is French.)
By the way, Americans have been conditioned to buy various flavors of rail passes for travel in Europe but unless you intend to do a lot of moving around by train, simply buying point-to-point one-way or round-trip tickets will probably be a better value and simpler.