I have just started to use monitor.us a free service it seems to do the job http://www.monitor.us/en/free-monitoring-features/server-monitoring
Free do follow back links for authors, leave reviews, articles, excerpts at whobuysbooks.info
This site is ok. works well with me"
I have had CUSTOMERS concerned about uptime. To keep ahead of the game I registered their site on http://www.serviceuptime.com/ and invariably (because it's a solid host) they are back on line in a short time. I get annoyed by emails sometimes but it's better to know ahead of time if they call!
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I used a desktop software called "Website Monitor", but after I solved some of my performance issues it seemed useless. Of course, you can need it sometimes, but it's interesting only if you make one-time payment.
Another free solution is using Google Docs, as it's exhibited in Life Hacker: