] QPX provides all of the information required to book and
] ticket any itinerary directly in a carrier's inventory
I've bookmarked this site along with orbitz and Yahoo Travel, as a very comprehensive price search site. It indexes every airline except for Southwest, and it also has a very cool graphical display. You can choose to show each flight on a bar chart, so you can quickly see departure and arrival and connection times. You can also sort by price, schedule and other factors, as well as getting advice on price differentials if your departure times are flexible by a day.
To my knowledge, orbitz still has the best multi-day search (3 days before and after), but the ITA display has been invaluable to me for checking tight connections through a variety of airports.