Planet Coaster is a hugely popular theme park management sim, created by the developers of RollerCoaster Tycoon 3. It's extensive building and creation tools have given it the chance to take on a second life, though, as a general world building tool.
Indeed, some of the most fun things to do with the game don't really involve building theme parks at all. One great example from this past week was the release of Star Wars - Death Star Strike on the Steam Workshop. It's a complete re-creation of some famous set peices from the Star Wars films in which it featured, including fights between Tie Fighters and X-Wings, a look at the hanger bay, and even a jump to light speed.
This ride is one of those truly innovative and awesome ideas that you just wish could exist in real life, but is unlikely to ever be a reality due to it's scope and size. These are the best rides you can build in my book - pure fantasy and imagination is what this game is all about.
The author of the ride has stated it's taken over 100 hours to build, albeit with some assistance, and that there's a strong probability that he will continue to improve the ride if the game adds new abilities which he can take advantage of.
You can grab the mod now over at Steam Workshop.