Crawl Stone Soup (http://crawl.develz.org/wordpress/) has graphical tiles based upon rltiles (http://rltiles.sourceforge.net/) with many modifications/additions.
Many of those artist making these changes have signed off their copyright, returning these tiles to a license similar to "public domain", CC Zero (http://creativecommons.org/publicdomain/zero/1.0/).
In an effort to promote sharing of this wonderful resource, this project seeks to periodically package these tiles from Crawl Stone Soup's version control, and provide them in a format easily consumable by other projects.
While every care possible has been taken to separate those tiles released into CC Zero from those who have not or can not be contacted, if you happen to see a tile incorrectly included in this export, please contact the maintainer (chris dot hamons at gmail dot com).
These tiles could not have been created nor re-licensed without the great work of the Crawl Stone Soup developers/artists.