Does there exist a plane ticket / airfare search and book webapp with the possibility to add a mail trigger, which will send me mail when a certain route gets under a user indicated price?
Looks like Hotwire has a trip watch feature.
Be your own travel agent
Trip Watcher acts as your own personalized travel agent. It gives you better control, value and flexibility when booking trips by allowing you to watch specific itineraries through email alerts.
Explore more travel choices
Tell us where you would like to go within a 60-day range you specify. We'll watch ALL those dates for you at once and find your low-priced travel days
Compare prices at a glance
View your watched trips and easily compare trips and prices for smooth planning.
Easily add trips from your search results by clicking "Watch this trip." Or sign in to "My Account" and add a new trip.