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.
There's even a handful of original coasters (hybrid coasters, single rail coasters, and alpine coasters), although you'll only see them in custom scenarios. Still cool that they've added two "modern" coasters that didn't exist when the games first came out (the first two I mentioned).