9. Kayak(free: iPad version, iPhone version)
Kayak's travel search is essentially the same as the iPhone app but its menus have been snapped together in a four-up display. Surprisingly, this ends up working rather well. One thing iPad users do miss out on though, is the neat pop-up effect the iPhone/iPod version has when jumping around the various search features. On the other hand, when browsing through the business end of Kayak, which is filtering through search results, the iPad version blows it out of the water. There, you can see close to three times the number of flights as well as what airlines you want to pick from--all on the same screen.