SLTag

SLTag is a utility to assign arbitrary semantic tags to local files.
Unlike other tools I've found, SLTag is extremely simplistic. Instead of using a database, SLTag stores tags in .sltag repositories.
A repository contains a folder for each tag which in turn contains symlinks pointing to the tagged files. Example:
dir/
|-- .sltag/
|   |-- tag1/
|   |   |-- 05168 -> ../../file1
|   |   `-- 24545 -> ../../file2
|   `-- tag2/
|       |-- 24545 -> ../../file2
|       `-- 58434 -> ../../file3
|-- file1
|-- file2
`-- file3
Here, file1 would have been tagged 'tag1', file2 'tag1' and 'tag2' and file3 would have been tagged 'tag2'.
The file's inode number is used as the symlink name so that broken symlinks caused by renaming or moving a tagged file can be fixed.
Querying tags is effectively just listing directory contents and requires no lookup at all. As such programs can, to a limited extend, use .sltag repositories without any kind of client.
Only querying for a combination of tags requires a bit of logic, which is done by SLTag.