OpenDiablo2
An open source re-implementation of Diablo 2
...The project is organized into a core engine (now evolving into a more generic 2D ARPG engine under Abyss Engine) plus toolset and support libraries. While still a work-in-progress, OpenDiablo2 has made strides: the main menu works, character selection and basic UI panels load, and movement in the first act is possible. The goal is eventually to support full gameplay — potentially allowing modders or developers to extend or make new games atop the engine.