Theme Park Builder 3D CAD- an ambitious community project to create a free amusement park construction game. You can build a huge park and then take a first person view and "ride the rides". Key features include user created content and expandability.
Project is dead.
Project renamed to TPB3D CAD for serious theme park design. It's not "dead" as someone pointed out. The lead developer had to take some time out and will return. TPB3D has 150+ rides modeled along with a growing list of other scenery models. A game version is in the planning for the RCT3 crowd.