Ammonite provides an easy-to-use interface to find tagged files quickly using the tags introduced by OS X 10.9 Mavericks. It also works with OpenMeta tags and databases of DEVONtechnologies' excellent product DEVONthink Pro.
The program takes a visual approach: tags applied to many documents will appear larger than tags applied to fewer documents. This helps you to drill down on the information you are looking for. It visualizes dates, file types and "nested tags" in separate clouds as well, providing a unified, intuitive interface for building even the most complex queries.
Ammonite is easy to use: left-click on a tag in the cloud to find documents having this tag applied, right-click to display documents not having this tag. Combine as many tags in your search as you need. The result is updated immediately. From here you can reveal or open them in Finder, DEVONthink or an external application. QuickLook is also supported.