Open source messaging system, fully scriptable in Ruby
Version:rat is an open source and easy to use messaging system, like irssi, that resides on the command line.
Operating System: Mac OS X
However, unlike irssi, it relies fully on plugins for all actual communication — meaning it's potentially capable of connecting you with anybody using any network.
rat is fully scriptable using Ruby, as well as being written in Ruby itself, so it's easy to hack and modify