Java Edition server software

A custom server is a Minecraft server that uses no derived code or components from the original Minecraft Classic or Survival server software, thus they are 100% free of Mojang code and restrictions. They are made as alternatives to the original server to add extra features, such as gameplay changes or to make fixes such as lag optimization. Custom servers have to stay updated to the current version of the client due to possible protocol changes, note this only applies to Beta at this time.

Features
Custom servers have a range of features, some of which are common between them. These features extend the functionality of gameplay, administration or even many other things never thought of before in the original server software.

Multiworld
Multiworld is the name of a most likely unintentional feature of the Minecraft client which can be used by a custom server. The first server to have this feature was Myne.

Usually on these servers they have a main world with portals to direct you to these other maps or by a command. They can also create new worlds and assign various properties to the world like ranks only in that world, only certain people can build, etc. This makes it possible to show old worlds while having maps for the public to build on and can grant more private stuff to prevent grief.

= Comparison = This is a comparison of available and maintained custom servers for both versions of Minecraft. Only servers that are actively maintained and/or supported will be kept on here, along with public servers. Forks will not be listed, exceptions can and will be made when and where needed.

Beta
This version of Minecraft only includes SMP (Survival Multiplayer) at this time.

Classic
For technical details on the Classic protocol, see Classic server protocol.

This version of Minecraft only includes Classic's Creative Multiplayer.

Forks
For more details on forks, see Custom servers/Forks.