Chatter
Chatter is a Windows Forms IRC client. It supports simple bots, some filters, and logging. Chatter uses NetIRC and NetIdent, so it can give you a good idea how to write a client using those libraries.
If you want to make your own chat bot, link to ChatterPlugin and plunk your DLL into the Robots directory. Have a look at the RandomSayings and StandardRobots projects in the source to see how the interface works. It's quite easy.
Chatter's license is Zlib-style.