Orbitz and Kayak both use them, check the wikipedia page on them for more info in that area[1]. ITA's real claim to fame, at least in communities like HN, is that they're one of the largest Common Lisp shops still around. Also my experience interacting with their employees has been that they're an exceedingly smart and talent bunch of people. Additionally, and especially if you lived in the Boston/Cambridge area, they're pretty well known for there rather challenging interview puzzles which used to be plastered all over the T [2]

[1] http://en.wikipedia.org/wiki/ITA_Software

[2] http://www.itasoftware.com/careers/puzzle_archive.html