I check travelzoo.com like every single day. I love it!

I use cheapflights.com and am signed up with every major airlines site. Most will alert you when they start a sale or air fare war. The best airlines to notify you are Southwest and United which included all their partner sites.

I would try Kayak.com it links to other travel sites to compare prices and can send you updates, there is a cool feature that shows you the average prices each month on a table to see what time is the cheapest to go. Plus I have also found that if you book the flight and hotel together it is a lot cheaper than separate.

I use http://hotwire.com all the time! They will let you submit alerts for specific locations and if they find a deal they'll email them to you. They have great discounts on hotels, flights, and car rentals ($15 a day in Miami...) They'll also let you know when the best time of the year to travel to a specific place is!

I use mobissimo.com I plan the greatest, cheapest trips from that site. I recently went to Turkey and found the lowest ticket possible (I'm from Hawaii, so this is literally halfway around the world), I would check 10 different sites, every day to see if the prices fluctuated and mobissimo seemed to be the cheapest one still yet.