Fixes bugs, adds proper widescreen support, enables modern Windows OSes compatibility, includes some minor content restoration, improves melee combat system, and does much more.
Found on WebArchive. Developed by WhiteMagicRaven.
Visit it for more details. The changelog is inside the archive.
Works with any version (includes all the official/unofficial patches and is cumulative).
To install, unpack the archive into Heretic II's (clean) installation directory and overwrite, and then launch the game as usual.
Starting a new game is obligatory.