As a trainer (among other roles) I have an interest in such online browser-based programming sites and have tracked some over time, and blogged about them, including repl.it. In no particular order:
Codingbat, Progress Graphs and Michael Jordan:
https://jugad2.blogspot.com/2013/02/codingbat-progress-graph...
Online Python Tutor looks quite interesting:
https://jugad2.blogspot.com/2013/03/online-python-tutor-look...
Online turtle graphics in Python from Runestone Interactive:
https://jugad2.blogspot.com/2013/02/online-turtle-graphics-i...
glot.io, an open source pastebin with runnable snippets:
https://jugad2.blogspot.com/2017/04/glotio-open-source-paste...
repl.it, online REPL for many languages, and empythoned:
https://jugad2.blogspot.com/2013/03/replit-online-repl-for-m...
And while I'm at it, a plug for my Python and other training courses (page updated recently):