If you live in or near Boston, Chicago, Los Angeles, Miami, New York (EWR), Pittsburgh, San Francisco, or Washington DC check out wowair.us. One-way flights range from $99 to $500 depending on dates. They usually fly once or twice a week so you have to plan your travel around their schedule. Seat selection ($10) and extra legroom ($40-$120) are extra on each leg of the flight. Checked bags are extra and some of the layovers are extensive (12-15 hours).
If you are traveling from Boston, Hartford, Denver, Fort Lauderdale, Orlando, Las Vegas, Los Angeles, New York (JFK, SWF or EWR), Oakland, Providence, San Juan, or Seattle Norwegian Airlines may be the ticket at norwegian.com/us. It offers fares for as little as $114 each way on flights to numerous destinations in Europe and occasionally have flash sales for even less. In the meantime, check out their low fare calendar for more deals. In addition to the base price, seat selection, checked bag cost about $90 one way. What is truly interesting if they have coupon codes so if you watch the sales you can get an even bigger discount on the total cost of your ticket. Their planes are the 787 Dreamliners so your ride is quieter and climate controlled for maximum comfort. The extra headroom makes it feel roomier even though the seats are still the same small size.
Icelandair has similar deals and serves more U.S. airports, but their flights are not usually direct unless you are traveling to Iceland. Otherwise, regardless of your destination, a stopover at the Keflavik International Airport in Reykjavik will be on your itinerary.
Searching comparable dates with the “big” airlines – fares averaged $500 - $800 each way for the same dates and they still add on baggage fees, reservation fees Other “low cost” airlines including Condor, Airberlin did not offer any discernable fare discounts.
So, until Ryanair ups their game and comes across the pond or United Airlines reduces fares to get more business after their most recent episode of “seatgate”, WOW, Icelandair and Norwegian are your best bets for low fares to Europe.