BungeeCord
Layer 7 proxy designed to link Minecraft servers.
BungeeCord is a sophisticated proxy and API designed mainly to teleport players between multiple Minecraft servers. It is the latest incarnation of similar software written by the author from 2011-present.
Information
BungeeCord is maintained by SpigotMC and has its own discussion thread with plenty of helpful information and links.
Source
Source code is currently available on GitHub.
Binaries
Precompiled binaries are available for end users on Jenkins.
(c) 2012-2017 SpigotMC Pty. Ltd.