Simon Express is the essential site monitoring tool for Mac OS X. It checks websites and servers for changes or failures, and notifies you via sound, speech, or other means. You can use it to track updated sites, and to alert you when an important server goes down or recovers.
Simon is very versatile. It can be used to monitor your own website and servers, track posts and new comments on your or friends' blogs, check for Web mail, get notifications of updates to favorite news and entertainment websites, keep an eye on auctions, and many other uses.