DEVONthink - DEVONtechnologies develops DEVONthink, DEVONagent, and other Mac and iOS apps for document and information management and web research.
TaggedFrog - Based on the convenient drag ’n’ drop technique, TaggedFrog allows you to organize your files...
Zotero - Zotero is a free, easy-to-use tool to help you collect, organize, cite, and share research.
Tagger - Tagger is a small application for OS X that can be used for quickly adding arbitrary textual tags...
HoudahSpot - Advanced File Search Utility for Mac OS X
Leap - With Leap you find things based on your natural memory of that file.