Signing up for Yapta.com just saved me $300+! I had registered a travel-agent-issued ticket to France for late May with Yapta several months ago. They watch the price of purchased tickets and notify you when the price drops below a certain level (they'll also watch non-purchased possible flights for you). Yesterday my ticket's price dropped dramatically, they notified me, and so I called my airline. They directed me to my agency for a credit, but the agency was VERY UNHELPFUL and NEGATIVE. So I called the airline back, explained the situation, and an extremely helpful person arranged for me to receive a voucher for #300+ for a future flight. I'd prefer a credit but she said they no longer give those. Now I can afford to take that day-long cooking class I've been wanting! So, check it out.
I've used Kayak.com the past couple of trips. Works for me and there's no additional software to download. It already saved me $400 for this trip (enough for our train passes).
A follow-up: no additional software needed with Yapta.com either, just sign up online. And, no, I don't work for them! Just love the fact that I saved so much money on my ticket to France later this month.
Yapta has also saved me a total of $105 on two flights this year. ($90 on one trip and $15 on another.) I use them to watch ticket prices for me before I buy. It's definitely the best way to keep track of airfare price fluctuations and to find a deal.