Introduced in OS X Mavericks, tags are a useful way to organize files based on assigned keywords. Now, typically, you’d have to go through the Finder to view them, but this one weird—and decidedly obvious—trick will let you access your tag collections from the Dock.
Start by opening a Finder window with the sidebar visible (if you hide the sidebar, unhide it via View > Show Sidebar). Scroll down to the Tags section (and un-hide it if necessary), then drag any of the tags listed to the right-hand side of the divider in the Dock. Drop it there, and boom, you’ve got a a stack that lists all files with that tag applied to it.
if you right-click (or control-click) the stack, you can get at the standard sort and view options for stacks.
It isn’t a perfect solution—the stack has a visible (and awkward) .tag0 filename extension, and when you right-click, you’ll find a “com_apple_SearchSystemFilesAttribute” sorting option. These quirks suggest that tag stacks are an unfinished feature, but they’re functional regardless. Give it a try and see what you think.