To filter a list by tags:

Select a list in the sidebar or navigate into a project.

Click on a tag in the filter bar at the top of the Things window. If you click on None, only items are shown that have no tags assigned to them.
If a list contains items with different tags assigned to them, a tag filter bar is shown at the top of the Things window. This filter bar only contains the relevant tags that you need to filter the current list.
Select a list in the sidebar or navigate into a project.
Click on a tag in the filter bar at the top of the Things window. If you click on None, only items are shown that have no tags assigned to them.
You can stop filtering by either moving to another list or by clicking on All in the filter bar.