I've written about Yapta a couple of times in the past (see "Related items" following this post) and how it's useful for today's airfare climate. As a refresher, you use Yapta's browser add-on to track airfares by "tagging" (or "bookmarking") the exact flight itineraries that you want while searching on airline websites. Once a trip is tagged, all key flight information - including price, airline and travel dates - is automatically stored on the traveler's "My Trips" page at Yapta.com and users are subsequently notified via email when prices drop.
If prices drop after you've purchased the ticket, you are notified when you're eligible for a travel voucher or refund from their airline.
In the past, Yapta only worked with Internet Explorer. Well, good news to you Firefox (Windows and Mac) users: Yapta released their Firefox plug-in today. It's free if you want to give it a shot. You can get more info or download the Yapta plug-in for Firefox.
Yapta can be used to tag flights on the following sites: