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.
BungeeCord is maintained by SpigotMC and has its own discussion thread with plenty of helpful information and links.
Source code is currently available on GitHub.
Precompiled binaries are available for end users on Jenkins.
(c) 2012-2017 SpigotMC Pty. Ltd.