Elyse - The free version allows up to 30 tags.
Leap - With Leap you find things based on your natural memory of that file.
DEVONthink - DEVONtechnologies develops DEVONthink, DEVONagent, and other Mac and iOS apps for document and information management and web research.
TagFlow - TagFlow is an intelligent files manager that use tags for organize your documents.
Yojimbo - Yojimbo makes keeping all the small (or even large) bits of information that pour in every day...
TaggedFrog - Based on the convenient drag ’n’ drop technique, TaggedFrog allows you to organize your files...