Repl.it: online environment for interactively exploring programming languages
repl.it
repl.it
Or when someone compiles Ruby 1.9 to JS, I will add it.
rlwrap[1] is not an "online" (ie. browser-based) repl, but it can be quite useful in a shell.
I would imagine that other languages like C could be brought in if they could be compiled using emscripten. However, people have had problems bringing clang and llvm into emscripten: https://github.com/kripken/emscripten/issues/579