I am using Pingdom and would like to use another services to double check on accuracy. The chance of having 2 services reporting false positive are very slim.
Keep in mind that a "false positive" may not be "false" -- if Pingdom or another service is showing your servers as "down" -- it may be that the route that they are using to get to you is experiencing issues. This may be affecting a portion of your customer base, so it may be important to take note of such an issue and work to resolve it.
Also, if it's that important, it may be worth spending a few dollars on a reliable service.
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BinaryCanary is extremely good. I personally love the Nagios OpsView setup - that's only really good if you have somebody at a desk 24/7 to watch it though. There is a nice FireFox plugin to send you alerts
My Quick Search shows 1 minute polling will indeed cost money. I have been using pingdom free without knowing the market price.
My closest search was BinaryCanary, cost $5 . month and offer 1 minutes polling. However it offers no Twitter or iPhone Apps. Generally i find Twitter / iPhone Push Notification much more reliable then email.
The next candidate would be uptimerobot. 5 minutes interval, and it is free.
We haven't found anything better than Panopta. The signup page seems to work ok for me - maybe drop them an e-mail at [email protected] and see if they can register an account for you manually.
Thanks sirius, it looks like a recent update to our website caused one of the paths to the signup page to break. This has been fixed and in no way interfered with our monitoring and alerting. iwod, if you run into any trouble signing up, PM me or email [email protected] and we'd be happy to help.