Is there too much text on your page? Do you need to allow users to un-clutter an interface, or hide some of the interface until your users need it? Don't go and grab any extra JavaScript, Drupal can do collapsible fieldsets out of the box!

However, there are some steps involved:

1. You MUST use a <fieldset> with a <legend> inside, the jQuery won't work right without it.
2. You have to enclose any content inside the fieldset in some sort of HTML tag. The styling will break otherwise
3. In order to get this to work, you have to include the collapse.js file from the misc/ directory.

You might think collapse.js is included all the time, but Drupal is smarter than you. This file is currently only included when a Drupal form has a fieldset in it. The upshot is that you can include it manually.

The "best" way to include the file is by using drupal_add_js() - placing this function call in a module is the path of least resistance. If you'd rather place this in your theme, you have a few extra steps. You'll need the information behind this link: Can't Add CSS, JS, RSS Icon Or Set Title Or Messages In Preprocess Page?

Now you need to create the fieldset itself:
<fieldset class="collapsible"><legend>Click to Collapse</legend><p>This is how you collapse code. Now what?</p></fieldset>

Example:

If you add the class "collapsed" as well as "collapsible", the fieldset will start out in that state.

Example: