One of the servers that I like recently allowed me to build a maze around the spawn. It was originally just to annoy people, but it evolved into a griefer deterrent. Most griefers would rather change servers than go through a maze. Plus, they are just plain fun.
While I made it, I realized how much work it was, and how much easier it could be. From that came MazeGen. Run MazeGen on your server map file and it will make a copy with a 3D maze around the spawn point.
As I cannot get a server working, this remains only half-tested. (I looked at the map in a Minecraft map editor) Any testers are very welcome, and your feedback will be addressed.