ahTellMe alerts you when something you want is available on the AH
I'm not sure if this is the correct board to post this, but it seems to be the closest I can find.
This is a World of Warcraft companion iOS companion app, not an add-on, but it is extremely useful for WoW players!
Tell ahTellMe what you're looking for; It will check the auction house for you and notify you when it has found it.
All the searching is done by the web service, so you don't need the app running on your phone all the time.
I hope nobody is offended by my posting this here - I have written this app to help wow players like myself.