Leap - With Leap you find things based on your natural memory of that file.
DocFetcher - DocFetcher is a portable German/English open source desktop search application.
DEVONthink - DEVONtechnologies develops DEVONthink, DEVONagent, and other Mac and iOS apps for document and information management and web research.
Everything by Voidtools - Everything. Locate files and folders by name instantly. Everything. Small installation file. Clean and simple user interface.
Elyse - The free version allows up to 30 tags.
Agent Ransack - Agent Ransack is a tool for finding files and information on your hard drive fast and efficiently.