macOS Mavericks officially brought tags to the Mac. Ammonite provides an easy to use interface to find tagged files quickly. It also works with OpenMeta tags and databases of DEVONtechnologies’ excellent product DEVONthink Pro.
Ammonite visualizes the tags used in a database or on your filesystem. Tags applied to many documents will appear larger then tags applied to a few documents. This helps you to drill down on the information you are looking for. Quick Look works like in Finder: Select a document and hit the space bar for instant previews.
Tag hierarchies are a unique feature of Ammonite which allows you to group tags by categories. An example: you have many ebooks and want to find all books by a given publisher? Tag your books from Manning with "Publisher>Manning" and the ones from O'Reilly with "Publisher>O'Reilly". Ammonite will create a dedicated cloud of publishers on the fly. There is no limit on how deep you nest tags.