CheapTickets.com - http://www.cheaptickets.com/
Discount Fares - http://www.discountfares.com/
Expedia - http://www.expedia.com/
Mobissimo - http://www.mobissimo.com/
Orbitz - http://www.orbitz.com/
PriceLine - http://www.priceline.com/
SideStep - http://www.sidestep.com/
Smarter Travel - http://www.smartertravel.com/
Travelocity - http://www.travelocity.com/
TravelZoo - http://www.travelzoo.com/
Uniglobe Travel - http://www.uniglobe.com/
Yahoo Travel - http://travel.yahoo.com/
More methods of transportation and reservations -
http://directory.google.com/Top/Recreation/Travel/Transportation/