I mostly work in the front-end web-development sections of [main], but sometimes branch out into the back-end aspects. For the front-end questions I find such sites as JS Fiddle and JS Bin invaluable for understanding problems and questions, not all questions or answers require a live demo, but some do.
Today I was trying, and frankly failing, to find a site to demo PHP code (though I'm sure I've come across such a site before, somewhere), so I thought it might be useful to have a larger repository of useful demo-sites for other languages, if only to save my future Google-fails.
So, perhaps one answer for each language for ease of searching (should the questions become a genuinely useful resource)?
(I couldn't find anything similar currently on the site, though that could, of course, mean that such a question has already been deleted...)