Spencer Museum of Art

Although I visited this place about a month ago, it still stands out in my mind as one of the coolest, and most innovative builds that I have seen.  When you teleport in, you might be confused to find yourself standing around a whole 'lotta nothing. That's because the majority of this build is slightly above ground level.  Looking up, you'll see a network of tunnels with interactive glass window entrances. When you click the glass, it will open and you will find yourself in a chamber that has the beginning of several more tunnels. Now, here is the real cool part that I haven't quite figured out: Every 10 minutes or so, the tunnel segments just randomly fall to the ground, and then they slowly rebuild. It's really amazing! In addition to the network of tunnelry to be explored, you can find quite a treasure trove of bizarre gems spread around the sim. Whether its strange buildings, eyeballs growing from plants, or floating sheep, there is something for every-weirdo!

Overall, Spencer Art Museum seems to have a rustic steampunk feel with organic elements laced into it. It's a really lovely balance of hard and soft; a symmetric theme.
ALSO! You even get souvenirs: An awesome hat that is a replica of the build, and some floating sheep!

http://maps.secondlife.com/secondlife/Spencer%20Art%20Museum/57/36/22