Modifications and Add-Ons

Ax-Crawl
Variant focused on UI expansion. Screenshots, developer notes, source, and executables.
http://axcrawl.berlios.de/

Crawl 5.x
Project home for planned full-scale rewrite. Developer snapshots available.
http://sourceforge.net/projects/crawl5

Darshan's Crawl Miscellanity
Monolithic patchset, description, and sample Windows executable.
http://www.angelfire.com/trek/mazewest/

Dungeon Crawl Alternative
Offers easier game start and a more difficult ending. Source and summary of changes.
http://www.interq.or.jp/libra/oohara/crawl-alternative/