In his first accepted submission, MaxShaw
writes "repl.it is an online REPL that supports running code in 15+ languages, from Ruby to Scheme to QBasic, in the browser. It is intended as a tool for learning new languages and experimenting with code on the go. All the code is open sourced under the MIT license and available from GitHub."
(previously used to build Doom for the browser
). All evaluation occurs client side, but saved sessions are stored on their server.