Tagadelic produces a word cloud using your data. You work out a way to tag or categorize data then pass the data to Tagadelic. Tagadelic creates the cloud.

Taxonomy Cloud creates a cloud display from taxonomy but uses Flash. If you want to create tags at the node level, adding taxonomy could be complicated. If you already use taxonomy then you are already rolling and just need Taxonomy Cloud to add the rock (or the 1980s lycra flash).

Views Cloud does what Tagadelic and Views Tagadelic do in combination. Choose your own poison.

Tag Clouds is a refined yet simpler alternative to Tagadelic.

Related terms does something similar but does not have a demo or screenshots.

Noticed another module: cctags.

Module MD WordCloud is a very pretty solution.