Java/JavaFX/Kotlin Game Library (Engine)
Arianne is an engine to develop multiplayer online games like Stendhal
Game engine to build text adventures.
GemRB is a portable open-source implementation of the Infinity Engine
Action RPG that's easy to mod
Cross-breeding a 3D FPS with a fantasy RPG in the world of Hexen
Heros of Jin Yong 3D Remastered
A free 3d game for PC that does not limit the player. Fight,Build,Gain
General Purpose Engine for Graphic Adventures.
2D Game Engine
RTSish Game Engine
A RPG maker engine using Pygame
A fast, scalable, distributed game server engine/framework for C++
Opensource Game Studio
Real-time RPG for PC/tablet/phone; multiple quests and random dungeons
A fantasy RPG.