www.montastic.com lets you keep an eye on up to 100 sites. It checks every 10 minutes or so and alerts you if the status of your sites changes via email. You can even monitor it via RSS or by using Yahoo widgets. But email works just fine for me. The best part about it is that it’s brain dead easy to use.
Edit: I am not as impressed with montastic now that I have used it for a while. I have received several false alarm emails when the site isn’t down. And it seems to take it a long time to figure out that the site is back up. I usually get an email the day after I fix the site telling me that everything is OK. Not so helpful, but still not bad for a free monitoring service.