What is the best low- or not-cost web site monitoring service?

Question by alsargentsf: What is the best low- or not-cost web site monitoring service?
I’d like to monitor my web application using synthetic transactions to ensure that it’s up and running properly. Some requirements:

– Check site every couple of minutes, 24/7
– Monitoring service itself should be redundant and reliable
– Send notifications via email
– Provide a data feed via RSS, Atom, XMPP, or something similar, so that we can use the data for our SLA reports that we publish to our customers.

There are tons of services out there — Pingdom, OneStat, and all the services listed at http://bit.ly/44shVs.

My question is, which is the best of the bunch?

Best answer:

Answer by Elpie
I really like Nagios http://www.nagios.org/
It won LinuxQuestions.org Members Choice for Monitoring Applications in 2007. It is, however, one you install on your own server so redundancy is a matter of how you set your server up.

Some good alternatives, all free:
http://www.montastic.com/ – it sends email notifications, has RSS and also offers a Yahoo widget. Downside is it checks every 10 minutes, which is a bit too long with a mission critical site.

http://www.siteuptime.com/ (the free service is pretty basic but there are optional pay-for services on top of this that are reasonably priced)

http://mon.itor.us/ – this one was recommended to me but I haven’t yet tried it.

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