Apple introduced document tagging in Mavericks, but you can usually only access those tags through Finder. Macgasm points out that while it’s a little clunky, you can add those tags to the Dock.
All you need to do is click and drag a tag from Finder to the right-hand side of the divider in the Dock. It will generate a stack just like you’d get with a folder, albeit with an awkward TAGO file extension. Still, if you use tags, it’s a handy way to access your them.
Show Tags as Stacks in The Dock [Macgasm]