Automatically run JavaScript code upon loading a specific website

You can use Tampermonkey for Google Chrome

Tampermonkey is the most popular userscript manager for Google Chrome.

Greasemonkey for Firefox:

Allows you to customize the way a web page displays or behaves, by using small bits of JavaScript.

You can use dotjs.

dotjs is a Google Chrome extension that executes JavaScript files in ~/.js based on their filename.

If you navigate to http://www.google.com/, dotjs will execute ~/.js/google.com.js.

This makes it super easy to spruce up your favorite pages using JavaScript.