OpenCraft is an alternative Minecraft server, providing more advanced features than the official distribution.
Minecraft’s official server implementation is fairly basic and does not have capabilities such as scripting and griefing preventation. OpenCraft aims to provide all of the features of the Minecraft server with this added functionality.
OpenCraft is written in Java and is built upon Apache MINA, SLF4J and XStream.
This is a simple python script that wraps around the official minecraft.net server. It implements a plugin system based on the parsing of log output. It also includes a remote control available via TCP, in addition to automatically updating the server when updates are released.
The newest addition to OpenCraft is NBT-Edit, a named binary tag editor. Here you can see it editing a player's inventory in Minecraft Alpha. It is very much a work in progress, but the source is available at http://gitorious.org/opencraft/nbt-edit for the brave ones.