OnWebChange - Tracking Amazon product prices (open in new window)
OnWebChange is intelligent enough to look for numbers and keywords so you can test for useful cases like when a stock price is less than a certain amount, or a date is within 1 months time or a news headline contains the word "Obama".
For more advanced users there's even a callback with configurable login variables so you can automatically adjust your own website content or make any other type of call.