Project Zulu is a minecraft mod that attempts to add flavor to minecraft by adding unique mobs, blocks, and structures to the overworld. For example a new block Nightbloom is added which changes form and glows at night. A common structure found is the labyrinth, showcased below, that contains two types of monsters: Minotaurs and Haunted Armor.
The Official wiki contains specific details on all blocks, structures, and options provided by Project Zulu. The Minecraft Forum Thread is a good place to be kept up to date with the latest version. Also via Blog or Twitter.
This video illustrates a procedurally generated structure, the Labyrinth, and the monsters, Minotaur, that can be found within its halls.
Source Code and Licence
Source code is hosted on Github Here. The mod is licences under Apache 2.0, visit the github project page for a copy.