Mark V Quake engine. It's not an official repo. Cloned to Github for convenience.
Version of engine cloned here: Mark V v1099 Revision 4
Official website: http://quakeone.com/markv/
Downloads and source code: http://quakeone.com/markv/builds/
Link to v1099 Revision 4 source code archive: http://quakeone.com/markv/builds/1099_mark_v_windows_revision_4.zip