OpenRCT2 is the open-source adaption of the classic hit RollerCoaster Tycoon 2. Including online multiplayer, fast-forwarding and numerous bug fixes.
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For best results, you should have the full game of RCT1 (Deluxe) and RCT2 (Triple Thrill Pack). Both can be bought on GOG or Steam, frequently on sale.
For a still-functional-but-not-ideal experience, you can get started with the RCT2 demo, or with RCT Classic. But again, above is recommended.