Crawl-tiles
Crawl Stone Soup (http://crawl.develz.org/wordpress/) has graphical tiles based upon rltiles (http://rltiles.sourceforge.net/) with many modifications/additions.
Those 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.
These tiles could not have been created nor re-licensed without the great work of the Crawl Stone Soup developers/artists.
Many examples of their use can be found on google images (http://images.google.com/images?hl=en&q=crawl+stone+soup)