borked

Munin is great. The http_loadtime plugin will monitor this for you and alert you when the site is loading slowly. Everything in munin is really easy to configure and will email you when "warning" or "critical" limits have been reached, from disk I/O, space, mysql, apache, anything.
Very simple to setup

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For coding work - hit me up on andy // borkedcoder // com
(consider figuring out the email as test #1)