Is there a way to make have Safari run some javascript load after each loaded page? I want to configure Safari to run a piece of javascript after each page I visit. The same peace of js should be run after any web page I visit is loaded. How can I configure Safari for this?
Yes, Greasemonkey or "User scripts" were the ones that popularized this.

For one option on Mac/Safari, please see http://www.macupdate.com/app/mac/20718/greasekit