Tagpad lets you use hashtags in your own notes.
Working on any plaintext document, it features an autocompleter to ease the insertion of tags, and multiple ‘tracker’ windows which let you ‘follow’ your tags and quickly jump to them.
For example, you can mark lines with tags such as #todo, #james, #suggestion, #idea, #bug or #important; and then follow ‘#todo and #important’ for a filtered view of matching lines.