OpenRCT2
An open-source reimplementation of RollerCoaster Tycoon 2 that expands the original with new features, rides, and multiplayer.
OpenRCT2 is an open-source reimplementation of RollerCoaster Tycoon 2, the classic theme park simulation game by Chris Sawyer. It runs the original game's scenarios and saves while adding dozens of quality-of-life improvements, higher resolutions, and a completely rewritten codebase.
Key additions over the original include multiplayer support so you can build parks together, custom scenery and ride objects, additional game mechanics like supporting loops and multiple trains per station, and a powerful scripting system. OpenRCT2 also fixes long-standing bugs and removes the limits that constrained the original game.
It is licensed under GPL-3.0 and is completely free. OpenRCT2 needs the original RollerCoaster Tycoon 2 game files to play (you must own the game), but everything else is built from scratch and open source.