Niconi Commons is a site where artists can publish works for use in e.g. remixes or covers.
For example artists may publish a vocal song as a video on the main Nicovideo site, then post the karaoke track on Niconi Commons so other artists can perform covers, which will be then linked back to the original track.
The materials can generally be downloaded for free, although you have to have an account on the site and you have to click through a license agreement provided by the artist.
An example URL: http://commons.nicovideo.jp/material/nc216831