buildawall - a wall builder for Minecraft. It builds a wall around your world.
Minecraft worlds are driven by a seed, and Minecraft creates the world from the seed as you visit new parts. However, upgrades to Minecraft can knock the world builder out of whack, and create jarring discontinuities where the old world ends and the new world begins.
Buildawall puts a wall around your existing world, creating a psychological and in-game division between old and new.
By default the wall is white wool with a core of indestructable bedrock. It's not-quite maximum height or depth, so you can climb over it or burrow under it, though that's still a challenge. By tweaking the script you can change the wall material, and make it destructable if you want.
These instructions are for Mac OS X. Ensure you have a working Python 2.x release.
Clone and install pymclevel (it's not in PyPI, sadly).
git clone git://github.com/mcedit/pymclevel.git cd pymclevel python setup.py install
Clone and install buildawall
cd .. git clone git://github.com/sixohsix/minecraft-buildawall.git cd minecraft-buildawall python setup.py install
Make a backup of your world
cp -pr ~/Library/Application\ Support/minecraft/saves/your_world_name ~/world_backup
buildawall ~/Library/Application\ Support/minecraft/saves/your_world_name/level.dat