Leap - With Leap you find things based on your natural memory of that file.
Scherlokk - Mac OS X application. Simple and user friendly, Scherlokk will find your media and missed files - quickly.
DEVONthink - DEVONtechnologies develops DEVONthink, DEVONagent, and other Mac and iOS apps for document and information management and web research.
Tembo - Tembo: Easy and Effective Searching
Elyse - The free version allows up to 30 tags.
Listary - Listary is a revolutionary search utility for Windows