JavaScript is the most readily available client-side scripting language for use in web browsers, and a lot of web sites use it to enhance their pages beyond the possibilities of plain HTML. JavaScript allows a lot of interaction between the HTML page and the viewer. This is especially true since you can add your own JavaScript to arbitrary pages to customise them in ways that the page producers did not necessarily intend (removing banner ads is a popular trick).
An easy way to do this are bookmarklets, small snippets of JavaScript that
you can install into your web browser by simply adding them as a bookmark
(the whole script is contained in a javascript: URL). Once you have done this,
they can perform all kinds of useful functions.
Just check out Jesse Ruderman's bookmarklet collection.
My two favourites are
The One That Gives You A JavaScript Shell
and
The One Where You Can Type CSS (these two apparently only work on Mozilla-based browsers, but there are many that work on Opera or MSIE as well). Amazing what can be done in such a small amount of code.
A way to automate the process of applying user-supplied JavaScript to web pages is Greasemonkey. Greasemonkey is a Firefox extension that matches a library of user scripts to the current page after it is loaded, but before it is displayed. Each such script can specify a list of sites that it wants to be applied to.
There are many user scripts available (and of course you can write your own), doing things like removing Google Ads, adding Google Ads, making text area input fields resizable, turning URLs into Hyperlinks, adding download links to embedded movies, removing links that have been visited before, downloading music files directly into iPods, cleansing news sites of Michael Jackson articles, or adding functions to GMail and various blog sites.