Chatter
Chatter is a Windows Forms IRC client. It supports simple bots, some filters, and logging. Chatter uses NetIRC2 and SecondLanguage, so it can give you a good idea of how to write a client with those libraries.
If you want to make your own chat bot, link to Chatter.Plugins and put your DLL into the Plugins directory. Have a look at the RandomSayingsPlugin and StandardPlugins projects in the source to see how the interface works. It's quite easy.
Chatter uses the Zlib license.