Windows: Freeware app NotiPage can track up to 20 sites, sending you a visible and/or audible alert when the page updates. You can set the app to monitor distant server accessibility, web site updates, and the appearance of specific words on site.
The utility can also be used to monitor a specific thread on a forums that do not have a notification feature. Setting up alerts is easy; enter the name and the URL for the alert along with any optional settings such as the monitoring rate and whether the alert should be when a specific page updates or when specific words are added to a site. It is simple to use and my test run alerted me of all Lifehacker updates within the last 24 hours. Of course, RSS is a great alternative for keeping track of regularly updated sites, but this could be handy when there's no feed on offer.
NotiPage is a free download.