TripBAM.com is a website that will automatically notify you by email if the daily rate at your selected hotel (or group of hotels) drops.
This works even if you prefer to book directly with the hotel chain, in order to get full points-and-stays credit in their frequent-guest program.
TripBAM does this by checking the rates every day on the hotel’s website. Note that it does not work on AARP, AAA, government, etc. rates.