There are so many great multi-search tools on the market right now. Some of the better ones are Booking Buddy, Opodo, Kayak and another great new one called Mobissimo. Mobissimo is unique in that it searches 60 domestic and international airfare sites to come up with numerous alternative prices that may not show up on domestic sites alone. Here's a list of airfare sites that Mobissimo searches (from it's website):

ANA, ATA, Aer Arann, Air Arabia, Air Asia, Air France USA, Air New Zealand USA, Airfare, Airtran, Alaska, Alitalia USA, Aloha, American, Amex, Anyway, Bahamasair, Basiq, British Airways, CanJet, Cathay USA, Cheaptickets, Continental, Delta, Destina, EBookers, EasyJet, Expedia.fr, First Choice, Frontier, Germanwings, GoVoyage, Hellas Jet, Iberia USA, Independence Air, JetBlue, Jetsgo, Jetstar, Korean, Midwest, Monarch, MyTravelLite, Northwest, Opodo, Orbitz, RyanAir, Singapore Airlines, Song, Sterling, Thai, Thomsonfly, Travelocity, Varig USA, Virgin Atlantic, Virgin Blue, Virgin Express, Volare, Vueling, WestJet, Wizz Air, Zuji.

I did a couple quick searches with Mobissimo, and I was able to come up with the cheapest price for my searches on Mobissimo. The price was cheaper by around $25. I've seen many reviews where people have found great deals on Mobissimo, and others where they've found them on some of the other new multi-search sites. In short, give a few a try, and you should come up with pretty close to the lowest price for the airfare segment you need.

Give Mobissimo a try and let me know if it works out for you!