Leap - With Leap you find things based on your natural memory of that file.
Evernote - Bring your life's work together in one digital workspace. Evernote is the place to collect inspirational ideas, write meaningful words, and move your important projects forward.
DEVONthink - DEVONtechnologies develops DEVONthink, DEVONagent, and other Mac and iOS apps for document and information management and web research.
allTags - allTags is a free, tag based file management application.
Elyse - The free version allows up to 30 tags.
Tabbles - Tabbles use tags to organize and find files along with your colleagues.