A while ago I wrote a little tag cloud web part, I’ve updated that web part so that it has its own solution package and can be used stand alone.
After you install and deploy the solution, make sure you activate the tag cloud feature in ‘site features’
Lets take the example of adding tags to a standard events calendar.
First add a new column named ‘tags’ to the events list.
Add the tag cloud web part to the page and set the following properties:
You can specify the link that each tag will link to as well. By default it will link to the search center and try to search on the metadata property of the tag field. i.e: "/SearchCenter/Pages/Results.aspx?k={tagfield}:{tag}", using this format however you could link to any page and then maybe use a query string filter web part to pull the tag from the url.
The final web part looks like:
You can download the solution from here.