Europe's many low-fare airlines don't show up on major search engines, and with more than two dozen budget airlines flitting about Europe, they offer a lot of flexibility in jumping from city to city. Now, new search engines let you scan low-fare airlines more easily than ever, even hooking them up with flights to and from the US.
To get an idea of the crazy diversity of low-fare routes in Europe right now, pop over to en.airtraffic.eu. This very basic website shows you dozens' of low-fare airlines' routes on a map of Europe, and you can connect the dots to see how routes hook up. Low-fare airlines aren't just about cheap flights; they can also get you directly to a lot of small destinations like Perugia, Italy; Carcassonne, France, or Cork, Ireland that you might have otherwise had to take a train to. But up until recently, we couldn't find any sites that combined transatlantic flights with low-fare airlines to give us itineraries from the US.
That's changed now, with www.dohop.com. DoHop combines full-fare and low-cost airlines to give you routes that don't appear on other sites. For instance, for a flight between New York and Granada, most sites came up with an Iberia routing. Only DoHop found an alternative routing that also involved low-fare airlines Vueling and Clickair. On the other hand, more conventional sites like found an inexpensive British Airways routing that DoHop couldn't find.
For a trip from New York to Carcassone, France, DoHop built an itinerary with Continental Airlines and super-low-cost carrier RyanAir. No other site could figure that one out, because Carcassonne is only served by RyanAir, which isn't covered by other major sites. Even then, RyanAir doesn't share their fares with DoHop -- only their schedules -- so I had to pop over to RyanAir's site and use a calculator to get my total fare.
In other words, as we already know, no one travel site has all the answers. But it looks like DoHop has some answers the others don't have. But here's the catch: European low-fare airlines don't guarantee any connections with transatlantic flights. When DoHop comes up with a multi-airline itinerary for you, they actually send you out to each individual airline's site to buy separate tickets. You then have to complete three or four transactions to get all of your tickets, and if one plane is late, the next airline will take no pity on you. RyanAir is especially cruel that way. Low-fare airlines also usually have much stricter baggage requirements than international flights, so you should always check baggage rules before you book so you aren't slapped with a huge excess-baggage fee.
If you're just searching for hops within Europe, though, www.skyscanner.net is the most well-established low-fare search site. They have a great fare calendar and a useable interface, and they've been doing this for years now; I first wrote about them in 2005. Like DoHop, they're an aggregator -- they send you directly to airline sites rather than selling you a ticket themselves.
WeeFly.com (www.weefly.com) is another aggregator, but I don't see its advantages over SkyScanner and DoHop. Competitors WeGoLo.com and BravoFly.com are even less attractive, but you might as well include them in your searches. As actual travel agents, WeGoLo and BravoFly will sell you the tickets themselves, not redirect you to airlines' sites. But they'll charge you a hefty service fee for the privilege -- WeGoLo charges $20, and BravoFly charges €16. For this fee, you get all of your tickets at once -- no multiple transactions. But unless one of these sites turns up with a truly great price, I just don't see the advantage. (DoHop and WeeFly search WeGoLo as part of their aggregator roles, so you're covered there.)
Still can't get to your destination? There's one last airline trick you can pull from your sleeve. EuropeByAir.com is a consolidator that sells a large number of routes on smaller, but not "low-fare" European airlines for $99 plus tax per flight. Their "FlightPass" routes (opens PDF) include a lot of wacky ideas -- Amsterdam to Vilnius, or Innsbruck to Kristiansand for $99! -- but they're flights you're probably not going to find at that price elsewhere. EBA's FlightPass flights are much more flexible than traditional low-fare tickets (you can change them any time, for free) and they're usually paper tickets, which means a $20 shipping fee. But still -- it's another option.
Finally, Arthur Frommer would frown upon me if I didn't remind you that sometimes European railways can be as fast as planes, given the hassles and delays of flying nowadays. Along with the traditional rail passes available through www.raileurope.com, you can often find deep discounts directly on rail operators' Websites. The excellent Man In Seat 61 rail guide page has instructions on how to book through national rail sites.
If all this seems like too much work, consider getting a good travel agent. Balancing out multiple plane and train options is something a skilled travel agent can do well, and it will save you a lot of time.