I am interested in collecting many user-defined tags for songs (primarily popular songs). These tags need to be descriptive of the song, but not standard metadata (like artist or album), because that information is very easy to obtain.
Last.fm lets certain users add/create tags for songs; not 100% sure who is a "certain user", but I'm assuming that it is related to longevity/activity on the site.
Here's a list of song tags that are included in the Last.fm dataset contribution submitted to the 1,000,000 song dataset:
The Last.fm Dataset - Million Song Dataset
And of course Last.fm's API:
Topped off by the
track.getTags method call: