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.
Slingscold - Slingscold is a lightweight application launcher similar to Launchpad in Mac OS X.
Elyse - The free version allows up to 30 tags.
kupfer - Kupfer is a command tool heavily inspired by https://alternativeto.