I have been using this for a while on my servers. For some months, I had Pingdom and Uptime Robot monitor the same server. According to my experience, the latter has less false positives - with Pingdom, it sometimes looked as if it was monitoring its own network congestion, and not the availability of my websites.
Uptime Robot also has an API like Pingdom, but with the added benefit you can monitor up to 50 websites for free. The API allows me to almost never log in to the control panel, as I watch the status using my custom monitoring panels.
This. False positives are terrible. We monitor google & amazon along with our services. If us, google & amazon all look down, it's a false positive. Not sure why pingdom doesn't do this for us, but whatever.
Uptime Robot also has an API like Pingdom, but with the added benefit you can monitor up to 50 websites for free. The API allows me to almost never log in to the control panel, as I watch the status using my custom monitoring panels.