Okay, so the subject seems to describe just about any media player out there already, but I couldn't think of how else to describe it.
I've had an idea recently that I think would be incredible for media players. It may already exist, or it may not be possible to do; I don't know about the inner workings of how data is stored in audio files. Anyway, the tagging I'm talking about is different than, say ID3v2 tags for MP3s. The kind of tagging I'm talking about is more like the kind of tagging people do on the internet. Short tags that describe and categorize the music based on personal associations (emotion, feeling, genre, instruments, events, whatever).
For instance, a techno song might have the tags "energetic, techno, happy, graduation, Sarah" or whatever you personally associate with that song.
What I think would be great is if there was a media player that allowed you to tag songs in this manner, that could also create playlists on the fly from your tags. You could select the "instrumental, depressing, new age, techno" tags and it would create a playlist of all songs that had at least one of those tags. Or perhaps you could customize it even further and require that the song must have x number of those n tags to be included in the playlist.
Is there anything out there that already does this? If not, is this something that would be feasible to implement with current audio formats?