[Zope] Zope monitoring tools or services

We have 2 public zopes and one admin zope on a ZEO...all on the same Linux box. We have squid in front and POUND in front of that. Would like to monitor the sites and inform us when down....really down, not just when Webmin can't talk to one of them or they are restarting themselves. Has anyone had success with remote monitoring services? Or luck with specific tools or utilities in Linux for monitoring? Our Zopes go up and down...sometimes the 'site' stays 'running' (or thinks its running) but the backend servers are down and do not restart themselves. I googled for a few links but wondered if anyone had specific successes. thanks Allen