VCMI Project
Open source community rewrite of the Heroes of Might and Magic III engine.
From the VCMI wiki: The purpose of VCMI project is to rewrite the entire HOMM 3: SoD and WoG engines from scratch, giving it new and extended possibilities. We hope to support mods and new towns already made by fans but abandoned because of game code limitations.
VCMI is a fan-made open-source project in progress. We already allow support for maps of any sizes, higher resolutions and extended engine limits. Currently the game is playable, especially in single player mode, but there are still some bugs and missing features.
VCMI is not yet a standalone program, it uses files and graphics from the original game. You need to have a version of Heroes 3 to install the files from (such as the one provided by GoG).