BuiltWith - Find out the technology behind websites
StatusOk (Open source) - Get notified through Slack/email when your website is down
Wappalyzer - Wappalyzer is a technology profilers and leads data provider. Create lists of websites and contacts that use certain technologies.
Hyperping - Cheap uptime and performance monitoring with detailed reporting and flexible alerting
What CMS - WhatCMS.org looks at a variety of factors within a webpage to determine what CMS a website is using.
Get Notified - Get Notified when something happens