Heads up! This blog post hasn't been updated in over 2 years. CodePen is an ever changing place, so if this post references features, you're probably better off checking the docs. Get in touch with support if you have further questions.

Alex Zaworski has the CodePen Enhancement Suite:

A Chrome extension meant to enhance the CodePen experience.

It adds a variety of functionality and optional aesthetic changes to the site. For example, a nice UI for creating your own syntax highlighting theme.

Check out the images and GIFs in the repo's README for the complete feature set.