Is something annoying you about your favorite web site? Don't like something about the layout, the controls, some annoying box elements with uselless information, or something?
You can fix it, at least for yourself, without having to hack the web server by using Tampermonkey, a user script extension for almost every web browser out there.
User scripts are also called Greasemonkey scripts because the first browser extension to support them was Greasemonkey for Firefox. You can still get Greasemonkey, but I like Tampermonkey better because it seems to be getting work done on it lately and there's a version of it for almost every browser, even Opera.
First, install the Tampermonkey extension. Then, get a sense of how it works by visiting one or more of these repositories:
Treat these sites as though they contain malware.
Browse the scripts others have written. Search for the name of the web site that's annoying you to see if anyone else has written a script for it. You might find something you can customize, or you might have to start from scratch.
Write your own script please
Well, I hope that got your attention. Really the best thing to do is write your own script. That way, unless you have dissociative personality disorder and one of your alts is an evil hacker, you can be pretty sure that you aren't running a malicious script.
Tampermonkey has it's own built-in editor. Just hit the Tampermonkey button and select Dashboard.
To get a new script, hit the little
+ tab in the upper right. You'll get a nice template with an IIFE (Immediately Invoked Function Expression) that you should put all your code in to avoid global namespace pollution.
Your script can be as simple or complicated as you want.
A DOM reference will come in handy.
Someone wrote a nicer guide than mine which contains screenshots. It might be helpful too!