2004 saw an increasing amount of attention given to travel search tools, particularly those that simultaneously search multiple databases. In fact, Yahoo acquired FareChase in July and launched a beta in September.
The article: Search engines revved up for one-stop travel service takes a look at some of the new travel search meta search engines including Kayak (they just announced a deal with AOL), Sidestep (download required), Mobissimo (just out of beta), and Quixo.
UPDATE: Pam Parker reports that Kayak is about to launch a ppc ad campaign to help bring traffic to their site. See: Kayak.com Readies New Year PPC Campaign. I've used Kayak for several months and particularly like its use of "sliders" to help modify and focus result sets.