bot.py – GPL3 – simple, uses python sockets directly. docs here
supybot – seems more complicated than necessary
IRC Bot – the essential 15 lines of code, maybe too minimal
linky – simple (150 lines) using twisted, example extending it here
python irc lib – very extensive, simple example included, but maybe it is not so simple
another simple example with a detailed explanation
phenny – another complete solution, that is more complicated than i probably want… but it is a debian package.
pynfo – twisted-based complete solution