Orbitz finds best fares and is backed by the major airlines. They have several preset routings or you can build your own. With Travelbids you put together a package and let it out for 'bid'. At Priceline you specify a desired price and wait to see if an offer is made. Or you can bid in travel related auctions on several sites.
Several airlines post last minute specials that are only available through the internet.
Check them for that spur of the moment travel event. Some have registration for regular email updates. In addition you can usually access the complete schedules of these airlines from these sites. Airlines of the Web has cyberfares, links to airlines with internet specials and allows a database search and Webflyer also offers a database search updated weekly. We list some of the main airline special sites but there are many others.