From OpenRCT.net:

OpenRCT2 is a free open source version of RollerCoaster Tycoon 2. It is strongly based on the original game and therefore looks like, and plays like the real deal. The game is rewritten into a modern language allowing people to extend and improve the game. Just like what happened with Transport Tycoon Deluxe which got remade into OpenTTD through a similar procedure. OpenTTD now has thousands of new future-proof features and OpenRCT2 will walk that path too eventually.

The game is still in development and thus can be buggy – meaning it can (and will crash a lot). There is however a stable version (0.0.2) available to play more or less bug free, you do however need a copy of the original game. (You can buy it at Steam, GOG, or maybe even your local game store) You need this because OpenRCT2 uses the original codes if that part hasn’t been remade, and it also uses the original image files to keep the authentic RCT2 look we all love.

Extra content can be found in our Content Center where you can choose from hundreds of new rides, rollercoasters and scenarios! You can also download our OpenRCT2 Launcher which carries a lot of extra functions which aren’t (yet) available in the game like an auto-updater so you always play the latest version. Our Launcher also has functionality to sync the amount of played minutes to your OpenRCT.net profile, but even better, it can back up your saved game files to the Coaster Cloud.

-Runelaenen, OpenRCT.net

I wrote the launcher there, and I want to say:

If you ever played RCT1 or 2, and loved it, go try this!