RollerCoaster Tycoon 3 in the Sandbox (many pics)
If you'll indulge my narcissism for a second (you should all be used to it by now).
There's a lot of talk these days about sandbox games. Fallout 3, Far Cry 2, GTA 4. Of course, what you do in a literal sandbox is build and create, limited by your imagination. That's the other end of the sandbox game spectrum. Games like the Sim City series, The Sims, or RollerCoaster Tycoon 3.
I love amusement parks. I love the roller coasters, I love the themes and decorations. So the RollerCoaster Tycoon games have likewise appealed to me. RCT3 was a fairly solid evolution of the series, the first 3D version. While not perfect, it scratched an itch and wisely included a pure sandbox mode that let you build a park without worry of money. After finishing the campaigns, I started on a sandbox park of my own. At some point I got distracted and stopped, but when I got back from Disney World back in December, the urge to resume the park hit me and I finally finished it.
It may come as a bit of a surprise, but this game can require a pretty beefy machine. This park, for instance, gets roughly 5 to 10 fps in fullscreen mode, and much lower while windowed. This finished park takes over 13 minutes to load from file.
The double deck observation tower is the perfect place to take in the entire park. It rises 355 feet into the air and takes so long that some visitors complain. Naturally, their complaints are ignored.