How to Create a Tag Cloud Using a Custom Data Source
I promised to follow up a previous article with a few examples like this one. If you haven't already, and you are new to custom data sources, you should read that article before reading this one. This is provided as an example only; it is not necessarily the best way to implement a tag cloud.
If your site content includes a section with a Tag List field (zero, one, or more words or phrases separated by commas) you might want to display the tags along with their frequency of use. One way to do that is to just list the tags along with a count of all the section entries that contain the tags. Another is to list the tags without the count but set the font size in proportion to the frequency of use. I use the latter method, called a tag cloud, on my archives page. Either way you will need to extract from your database a list of all the tags used and a count of the entries that contain the tags.