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.
Recoll - Recoll is a desktop full-text search tool. Recoll finds keywords inside documents as well as file names.
Elyse - The free version allows up to 30 tags.
FileLocator - FileLocator Lite enables you to locate files on your pc.