HoudahSpot - Advanced File Search Utility for Mac OS X
TagSpaces - TagSpaces is an open source platform for personal data management. With TagSpaces you can manage and organize the files on your laptop, tablet or smart phone.
Elyse - The free version allows up to 30 tags.
allTags - allTags is a free, tag based file management application.
DEVONthink - DEVONtechnologies develops DEVONthink, DEVONagent, and other Mac and iOS apps for document and information management and web research.
Tabbles - Tabbles use tags to organize and find files along with your colleagues.