XML is unnecessarily complex but I think JSON also is.

IRC and MSNP (the early versions, before they started XML-ifying everything) are what I'd consider a good format protocol for the application - a mostly line-based, textual one you can use pretty easily without any client more complex than netcat. They are also extensible enough even without XML.

(Personally, I think it is absurd that a protocol needs to wrap a single text message, the most common form of message that would be encountered, in several times more bytes than the message itself.)