Here is my scenario.
- I pose a programming puzzle. It might be golf, or might be something else, but I don't specify the language.
- Some posts a answer that looks really good, but I don't have a working environment to test it.
Let us compile a list of free execution environments reachable over the internet.
One answer per answer, please, and list the protocol and host as well as as much detail on the available execution environments as possible.