I wanted to add a sidebar block showing the most popular tags on the site but didn't want to use any extra contributed modules. It turns out it's actually quite easy to do using Views.
- Download and install Views if you don't have it already.
- Create a view to Show Taxonomy Terms and click Create a block if you want the sidebar block.
- Click Advanced and add a Relationship to Taxonomy term: Content using Tags
- In Advanced under Other set Use aggregation to yes.
- Add the Taxonomy term: Name field twice. Set the aggregation settings of the first to Group results together and the second to Count.
- You can add a Prefix and Suffix of ( and ) so that the number of times the Tag appears is in parenthesis.