It started back in the day of the old #blade channel (efnet) for the online art group Blade Nation. The bot was called, I think, y0da-1. You could talk to the bot and it'd make fun of you.
Lately, bots seem to be getting a whole lot more functionality thanks to API's and XML-RPC and the like. Over in #virus on irc.lucifer.com there's the LogLady - Twin Peaks inspired - who, when voiced, records everything anyone says into a database that's searchable on the web. The channel also has googlebot, which was written in Python and uses the Google API to send queries and format responses.
But lately, spurred on by my quest to master Jabber and become a part of the social software community, I've been hanging out in #joiito on irc.freenode.net. There, lives the open source Python bot: Jibot. That's right, check the link and bask in the glory of what this bot can do. Then head on over and read Kevin Marks' entry on Social Botware, which does a damn good job of describing the channel's dynamic and how the bot has fit into it and ultimately helped shape it.
And if you're asking yourself why I'm awake and blogging so early, it's to vicariously attend the Supernova conference on decentralization.