PircBot is a Java framework for writing IRC bots quickly and easily. Its features include an event-driven architecture to handle common IRC events, flood protection, DCC resuming support, ident support, and more. Its comprehensive logfile format is suitable for use with pisg to generate channel statistics. Full documentation is included, and this page contains a 5-minute step-by-step guide to making your first IRC bot.
PircBot allows you to perform a variety of fun tasks on IRC, but it is also used for more serious applications by the US Navy, the US Air Force, the CIA (unconfirmed), several national defence agencies, and inside the Azureus bittorrent client. But don't let that put you off - it's still easy to use!