There are many alert systems around and web monitoring solutions but something really simple and useful for many is a web alerting tool that helps you alerting when a web page has been changed.
One of these free services is Change Detection. With Change Detection you can be alerted when a change is made on a web page. Not only by a change on the page size but also when the content changed, when new content is added or removed from the page.
Applications like this one are very useful for many purposes, if you came to this article looking for such product then you may have clear why this can be useful for you . In order to monitor a webpage, you just need to open Change Detection in your browser, add the URL that you want to monitor changes and then enter your email address. You will be requested to register to the site, which is not so bad since you can add multiple pages under the same account.
Once ready, submit the form and confirm your account. You will be asked to enter some other settings for the pages that you are monitoring, like the criteria to consider when the page changes and trigger the alert.