(See drupal.stackexchange.com)

Questions can have tags (e.g., "custom-modules" or "hooks").

Users can "follow" tags and get email when new questions with that tag are posted.

On user profile pages, the tags used are listed

On the main question listing page, a block is shown with recently used tags.

Rolling over a tag gives "vital stats" and the ability to follow the tag.

Users can browse a page giving "popular tags"