As pointed out in this post on the Zend Developer Zone, the tag cloud can be a valuable tool to any site using tags for its articles/posts. They also mention a tutorial to help you build one.

Well, it's actually so simple it's not really a tutorial at all so much as a snippet with a bit of explanation. But I've had some people ask how it's done, so here's how I do it. I tried to go through and add some comments about what's happening.

The code pulls the information from a backend database and calculates out what size each of the items needs to be based on the max and min values set in the script.