MockServer - Easy mocking of any system you integrate with via HTTP or HTTPS.
Hyperping - Cheap uptime and performance monitoring with detailed reporting and flexible alerting
HttpMaster - HttpMaster is a professional software tool for testing and debugging HTTP applications, primarily aimed at REST API applications and web services.
Suprvise - Know instantly when your sites are down.
Hoppscotch - Open source API development ecosystem
Oh Dear! - Website monitoring, without the hassle