PageModifier

Create scripts for modifing page elements.You can use this tool to execute custom Javascript codes for selected websites or for all the websites.

To use it, click the extension icon when you are in the targeted website, then enter the Javascript code or copy-paste to it if you have already written it somewhere. Click save so that this code will be executed every time you visit the page.

It will be helpful to provide some additional functionalities that the original website does not give to you. For example, you can create a script to automatically replay the video again on Youtube when it finished. This feature is not available on Youtube as far as I know.

The extension provides various configurations for scripts created. For example, you can configure a script to be executed for all websites, or you can configure it to be executed only when some URL patterns are detected.

The extension will save the scripts to your Google account's cloud storage. If you are enable Chrome to synchronize data between different machines, the scripts you have saved will be synchronized as well.

The extension supports jQuery codes if you are more familiar with it.

For security reasons, this tool only uses Chrome's storage API to save scripts. It has a limitation of how much data can be saved, as noted here: https://developer.chrome.com/extensions/storage