farecompare offers a addon to firefox that will check fares for particular route(s) for you a few times. FC subscribes to a feed, so when fares change the little FC icon in the lower right corner of firefox gets a red dot and flashes an alert.

https://addons.mozilla.org/en-US/firefox/addon/2040

Another similar tool is yapta, although I have not tried it.

https://addons.mozilla.org/en-US/firefox/addon/5966

Amex Travel and others provide email alerts when fares drop.

The problem with all this info is that sometimes the availability for the fare class the offer books into is not available.