HTTPBin

https://httpbin.org/

A simple HTTP Request & Response Service.

https://github.com/postmanlabs/httpbin

mostly unmaintained, but also functional. The httpbin.org server is wobbly sometimes.

https://github.com/mccutchen/go-httpbin

go-httpbin offers very similar functionality to httpbin, but written in go and can serve TLS. Much easier to deploy, so I've started using it in a docker container for testing httpsnippet, and I've contributed a few improvements so that it can be used in that setting.

Available on the web at https://httpbingo.org/

Postman serves postman-echo.com with a similar function, seems not to be open source though