This is currently only available in Showcase and only for the Widget Definition "Showcase: Latest items". Update: This is now available in the "Showcase: Featured items" Widget Definition as well
There is a new option (limit) called "Tags". This uses the xf:tokeninputrow which allows you to add 1 or more tags to the input.
Note: The ITEM must have all tags that are added to the input to be considered a MATCH. Eg, if you add the tags FORD and HOT ROD, only items that have BOTH of those tags applied to them will be fetched, not one or the other, but BOTH.
Here you can see that I've applied the tag "muscle car" to the widget, so only Items tagged with "muscle car" will be fetched.
And here you can see that the Latest Items has pulled several items that have the tag "Muscle car" applied to them.
Lets add a 2nd tag and see what happens. As you can see, I've applied 2 tags to this widget "muscle car" and "chevy", so it will only fetch items that have both tags applied to them.
And here you can see that the amount of items has bee reduced as there are only 3 items that are tagged with both "muscle car" and "chevy".