You know that clicking a Tag lets you view a list of subscribers who've been added to it. But what if you want to view a list of subscribers who don't have any Tags?
Here's how to find them:
First, create a new Segment.
Select All Subscribers as your first filter group:
Then, add a new filter group that matches none of the following.
Set up this new filter to group to include every Tag in your account.
Save your Segment, and voilà. The subscribers listed in this Segment are those who don't have any Tags.
NOTE: Since Segments always initially target all subscribers by default, the first part of this Segment with the "All Subscribers" filter technically isn't necessary. However, including it can be helpful for remembering what the Segment does.