DubCord 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 it's own discussion thread with plenty of helpful information and links.
Source code is currently available on GitHub.
(c) 2012-2016 SpigotMC Pty. Ltd. (c) 2016 SurvivalDub