I know this is an old thread, but in case anyone else discovers this thread while searching the internet for website monitoring software, we thought it would be useful to let you know about ours. We are a reputable UK software house and our server monitoring software is called Cyclops. It offers unlimited checks and SMS/email alerts for downtime, helping you maximise your site's uptime.
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And although this post is blowing our own trumpet, we do intend to generally contribute to the forum and to other threads!