Initially there was rltiles (http://rltiles.sourceforge.net/). This was/is a set of public domain 32x32 tiles for Crawl and Nethack. 

Over time these were extended, modified, and new tiles were added. Much of this work was done by the Crawl, and, later Crawl Stone Soup team (http://crawl.develz.org/wordpress/). For a long time, these changes/additions were under the "Crawl" license, unable to be used by most other projects. 

The wonderful developers/artists who work on Crawl Stone Soup have signed off their copyrights on the tiles enclosed in here, returning them back to a state similar to public domain (CC Zero, see LICENSE.TXT). They are free to use for any purpose. However it would be nice for those using them to credit the developers/artists, and state where you recieved them so others may find and use them as well.