(As of May 3, 2015)
It seems that Google has bought ITA, which is now Google Flights, and Microsoft has bought a bunch of other travel sites, whch are now all Expedia. The remaining player in the market is Priceline’s Kayak.com. Google will send you to a single airline site to book all of your flights, Microsoft’s Expedia acts as a travel agency, Kayak sends you to airline sites.
For really cheap travel, priceline.com uses an auction system, but you never know if you’re going to be routed through New York on your way from Seattle to Walla Walla.
Me, I travel as the crow flies.