Allows the connection of newer clients to older server versions
...Note that ViaVersion will be able to run best on either Paper servers or through ViaFabricPlus due to having direct access to client/server state and more extensive API.
A Minecraft world downloader that works as a proxy server between the client and the server to read & save chunk data. Download multiplayer worlds by connecting to them and walking around. Chunks can be sent back to the client to extend the render distance.
An alternative/custom Minecraft server, written in Java, compatible with the original protocol. OpenCraft is built upon Apache MINA, SLF4J, Jython and XStream. The aim is to have features not in the official server distribution, like antigrief/scripting.