Type

javascript:alert('hello world!')

into the location bar of your browser!

Congratulations, you're writing javascript! You can execute things directly in your browser like this, save them into bookmarklets to easily execute again, or turn more complex bits of code into a Greasemonkey script

You can also modify objects on the page – the following snippet will make the first image on the page 100 pixels tall:

javascript:document.getElementsByTagName('img')[0].height = '100';

Next: Intro to Bookmarklets