tag-not-ed is a TAGged NOTes EDitor.
This is the homepage of tag-not-ed, my favourite program. [You might have read this before somewhere else] Give it a try if you feel like it.
It is a system which allows you to create and manage text documents by attaching tags to them.
Later, documents can be retrieved by running queries on those tags (e.g., "show me all docs that deal with 'dogs' and 'cats'").