Realcraft is a custom (third-party) Minecraft server written in Realbasic by Jailout2000. It implements the Minecraft 1.0.0 server protocol. The server runs out of the console; it does not have a GUI to which you could control it.
The server is being written for Windows mainly but will also be compatible with Linux. I plan to support Mac OS X in the future, but currently it is not supported.