VCMI - it’s re-written from scratch, open source, multiplatform Heroes 3 engine with many improvements and mods manager. It requires some files from the original game though.
It’s definitely a work-in-progress title, but I have really enjoyed SuperTux Advance (AGPL-3.0 according to their github page). It’s like SuperTux but with sliding, more playable characters, and more power ups. As of now it’s probably my favorite OS game at the moment. -Ace