Free open-source website monitoring software

 User friendly
 Platform independent
 Can run standalone (also as a Windows Service) or on your infrastructure
 Monitor single web page, pages in sitemap and even your whole web site using spider
 Check not only HTTP result codes, but also page contents
 Find broken links on your website
 Microservice (REST APIs) monitoring - XML (using XPath and XSD) and JSON (using JSONPath)
 Periodic checking (built-in cron mechanism)
 Email notifications
 Statistics