We made one pretty easy to extend in python, it works with XMPP / gtalk :
http://gbin.github.com/err/
To give you an idea the minimum hello world is as follow:
from errbot.botplugin import BotPlugin
from errbot.jabberbot import botcmd
class HelloWorld(BotPlugin):
@botcmd
def hello(self, mess, args): # the chatbot will then respond to the command !hello
""" this command says hello """ # this will be the result of !help hello
return 'Hello World !' # this will be the answer