Airfare

• Kayak.com - supports multi-hop itineraries
• Fly.com - meta site
• FareCompare.com - meta site
• AirfareWatchdog.com - manual searches by actual people
• Momondo.com - Leaves no stone unturned (includes high-speed European train routes)
• Dohop.com - broad, impartial search
• Bing.com - price predictor
• STATravel.com - for students
• StudentUniverse.com - for students
• Hipmunk.com - sorts according to "agony" index (price, length of flight, number of connections, etc)
• InsideTrip.com - evaluates by legroom, aircraft age, on-time performance, etc
• Yapta.com - alerts you if rates dropped after you've booked