TMSU - TMSU is a tool for tagging your files.
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.
TAGXFS - Tagxfs is a semantic file system. It extends the user space file system to a tag based hierarchy.
allTags - allTags is a free, tag based file management application.
Danbooru - Danbooru is an advanced, tag-based image board system that is based on Ruby On Rails.
Tabbles - Tabbles use tags to organize and find files along with your colleagues.