install user customized JavaScripts (a simple cut/paste operation) into your user page's vector.js or monobook.jsskin. Installing to your common.js subpage will allow both skins to use a single JavaScript installation.
As you can see, tools are implemented in a number of ways. There are tools available for many different purposes, and with many different functions, and some that are multipurpose. Many of these are explained at Wikipedia:Tools/Browser tools, which includes a list of tools, for each major browser.
