Web-based REPL's for many languages
I only just recently learned the term “REPL”. It essentially means “command line”. You type code in, it runs, you see output.
Read, Eval, Print, Loop.
There is a console on CodePen as well, which can be useful because you can create examples that link to REPL output:
CoffeeScript offers their own as well:
repl.it is a a web-based REPL for a bunch of languages, including Ruby.
You can save your examples for later reference as well.
Skulpt lets you run Python entirely in the browser.
PythonAnywhere also has IPython, an online interpreter.
repl.it seems to be a good choice here again:
For the record, repl.it supports an impressive number of languages, so if you’re looking for a specific one, you might try there first.
Java REPL has a console that is as simple as it gets:
IBM makes a REPL for Swift. They call it a sandbox, which is a common name for these kinds of things.
Clojure is weird. I enjoyed TryClojure because it not only allows you to write/execute Clojure, but also walks you through learning it interactively: