US-centric: Keep obsessive tabs on multiple airfare trip price changes with the Yapta Tagger, currently available only for Internet Explorer. Here's how it works: after downloading the Tagger, you go to the various travel sites that Yapta has a relationship with (most airlines are covered, as well as Expedia and Travelocity) and dream up an itinerary. Enter in the necessary information and tag it to your My Trips —from here on out, Yapta will keep track of any time that travel package price changes.
You can decide how much you want to be notified of; for instance, you don't have to get tipped off unless the price change is above a certain dollar amount. It's a great way to keep track of airfare changes without having to, you know, actually do anything. Yapta Tagger is a free download, it only works with Internet Explorer at this time; however, you can use the site's interface to track one airfare price change at a time in other browsers.
Au - While Yapta doesn't work with any Australian travel websites, you still might find it useful if you're looking at overseas flights. You can view the list of compatible websites here.