HoudahSpot - Advanced File Search Utility for Mac OS X
DEVONthink - DEVONtechnologies develops DEVONthink, DEVONagent, and other Mac and iOS apps for document and information management and web research.
Elyse - The free version allows up to 30 tags.
TaskSurfer - TaskSurfer gives you all of the tools you need to manage things the way you want.
TagFlow - TagFlow is an intelligent files manager that use tags for organize your documents.
IDocument - Feel powerless of your documents chaos in your Mac?