Tagfs - tagfs is Fuse file system to organize your files using tags.
TagSpaces - TagSpaces is an open source platform for personal data management. With TagSpaces you can manage and organize the files on your laptop, tablet or smart phone.
TMSU - TMSU is a tool for tagging your files.
allTags - allTags is a free, tag based file management application.
TAGXFS - Tagxfs is a semantic file system. It extends the user space file system to a tag based hierarchy.
Elyse - The free version allows up to 30 tags.