What's a simple way in Google Chrome to insert Javascript into sites?

The answer was Tampermonkey.

https://chrome.google.com/webstore/detail/dhdgffkkebhmkfjojejmpbldmpobfkfo

Why not dotjs http://defunkt.io/dotjs/ ? It's local, you can version it with git, you can easily take it to another computer...

Another alternative that neatly solves the problem is Custom JavaScript for websites. You just need to install the extension, which takes around 2 seconds, and then you can immediately start typing your custom JavaScript for the specified website.

The extension automatically recognizes the current website, so all you need to do is write your code and click on Save. You can also easily import jQuery or your external scripts for convenience.