In the past, I used to use Pingdom and UptimeRobot. Now I mainly use HetrixTools. PS HetrixTools offer a free version if you're interested. Which paid & free website monitoring tools do you use?
Hyperspin monitors server uptime in real time. They have a list of hosting providers that they monitor.
Using SolarWinds Server & Application Monitor, you can monitor server uptime across your environment with integrated templates designed to let you monitor over 1,200 applications, operating systems, and devices in your network to help make sure your systems and services are performing optimally.