I just signed up to have my sites monitored by Panopta, and they are wonderful. They perform a wide variety of server checks, and notify me with a credible alert within a minute of my site going down. I was getting a bunch of random checks that didn’t actually resolve with other services. One of the things I like about Panopta is that they double and triple check the outage to make sure it is worth notifying me about.
If you run an online business that depends on consistent uptime, I would recommend Panopta. And the customer service is top-notch. Very happy to give them a shout-out.