Site 24x7 is a free service that lets you monitor a site's response time over time and sends alerts if a site is down or slow. You can set how often Site 24x7 monitors the site: the smallest interval is 5 minutes.
What's nice about this service is that you can get alerts by mail or SMS if something bad happens with a site: slow load time, most of the content is changed (defacement?), HTTP errors. Reports can be sent to any number of mail addresses and phone numbers, but you can only have 10 free SMS notifications per month.
The service is in beta and will continue to be free after it's officially launched, although it will have some limitations for the number of monitored sites.