OpenRCT2 is the open-source adaption of the classic hit RollerCoaster Tycoon 2. Including online multiplayer, fast-forwarding and numerous bug fixes.
"OpenRCT2 is an open-source re-implementation of RollerCoaster Tycoon 2 (RCT2), expanding the game with new features, fixing bugs and raising game limits."
A project I never see great brought up enough so here's some love to a great classic game that's been revitalized by fans.
They require you have the source files for the original game, which you can usually get for 1-3 bucks on gog or steam. Totally worth it for this masterpiece.