Clojure REPL at http://www.tryclj.com/ which has link to Github source code.

Clojure(Script) REPL at http://clojurescript.io with source code at https://github.com/ScalaConsultants/replumb

Myrddin has an online playground http://eigenstate.org/myrddin/

If you haven't heard of myrddin (from the site):

Myrddin is designed to be a simple language that runs close to the metal, giving the programmer predictable and transparent behavior and mental model. It also does strong type checking, generics, type inference, closures, and traits.

A few more classic and esoteric languages are available in https://repl.it/

Something we did, includes Swift and shell: https://codepad.remoteinterview.io

Also, for making your own such service: https://github.com/remoteinterview/compilebox

tmpnb launches temporary Jupyter notebook servers using Docker containers:

https://tmpnb.org https://github.com/jupyter/tmpnb

swi-prolog has a very-nice web IDE:

http://swish.swi-prolog.org/example/clpfd_sudoku.pl

MBED.org

C++ for embedded devices