What's the best way to know if your web server goes down? [closed]

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http://serverfault.com – Possible Duplicate: What tool do you use to monitor your servers? I noticed my website only got 8 visitors today, which means it probably went down very early this morning and I never noticed. Why it went down is another story. Ideally, I'd like to be emailed if my web server becomes unresponsive or does not return an HTTP 200. Is there either a cloud-based service (either free or pretty cheap) that can monitor your website? If not, is there a good free/open source program I can run on either a Linux or Windows machine that will monitor a website and email me if it goes down? T (HowTos)