Tagadelic provides an API and a few simple turnkey modules, which allows you to easily create tagclouds, weighted lists, search-clouds and such.
With the API you can build a module with a few lines of PHP, to turn anything that can be counted into a weighted cloud. Which can be presented to your users anyway and anywhere on your site.
With the turnkey modules, you can add a page that shows taxonomy-terms in a weighted cloud: terms that are used more often are bigger. Another module provides a page that shows article-titles in a cloud: titles from articles that are read more often appear bigger.
Tagadelic 2.x branch is work in progress and should only be used by people who plan to develop for and with it.
Tagadelic 1.x branch is stable for Drupal 6 but unstable for Drupal 7. End-users should use Drupal 6, 6.x-1.x versions. End-users are strongly advised not to use Drupal 7, 7.x-1.x of tagadelic; it has known bugs, yet is reported to work for some people. Use at own risk.