HTTPBin
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
https://httpbun.com
https://github.com/sharat87/httpbun
Similar to go-httpbin, but offers a neat mix endpoint where you can combine flags to create responses with the exact status, headers, body and latency you want.
Written in go