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.
Re: openRCT2: you will need a copy of the original RCT2 in order to play it - it doesn't include the assets due to copyright. The GOG version's assets will work for it if you don't have the disc.
OpenTTD however, has its own custom open replacement assets, so you don't need the original game.