On XBL there's many servers that people host which of course aren't really powered by servers, just people hosting lobbies. I once played in a "factions" server which was hosted 24/7 by some guy, he just left his game system on, posted the thread on MCForums and let users join. That's all there really is to it, I wouldn't call it a "server" of any type though.
Not sure if the same applies for PSN, however there is a subsection on MCForums for lobbies. (