Here's a good one. OpenACS
and Alternative Linux Architecture Labs
have collaborated (and for some time) to provide free HTTP uptime monitoring.
Set up is a breeze. First, you upload a text file to your server with the word "success" in it, and nothing else. Then you create your monitoring account, specifying a contact email address and one or more public URLs to monitor.
The service can either notify you periodically or upon server failure and server recovery. The reported check interval is about once every 15 minutes.
This is probably sufficient for non-mission critical sites. For those who require finer check granularity or more sophisticated reporting, there are plenty of other services
to be found. [Thanks to LifeHacker]