I dont know much more about free website monitoring tool but I have used one free trial of 7 days from one site which provide website monitoring. Its nice one.
I actually found one and its great, www.siteuptime.com Its free account checks the site every 30 minutes.
Hi, Check out ObservePoint.com for Web server monitoring tool. It's not a free but it's worth to get trial version of it as I came across that it provides best web monitoring services.
If you are still looking for website monitoring tool here is a free one 24/7 mon.itor.us I hope will help as well.
I use pingdom for free monitoring (checks every 1 minute instead of every 30 minutes). It sends me an sms when my site is down. 20 free sms alerts are included.
The only tool I know that sends you notification everytime something goes wrong on your site is SEO Cockpit. Its a website monitoring tool at its best. check it out http://www.seocockpit.com