PyScript lets you run Python scripts right in the browser, side by side with JavaScript, with two-way interaction between your code and the web page.

Run Greasemonkey Scripts in Google’s Chrome Browser What’s a browser without Greasemonkey? It’s a sad little thing that can’t do any of the cool stuff that Greasemonkey enables.

With trinket.io, you can run python scripts in your browser. Even cooler is that they also run on your phone.