Sugar-Cube is flexible IRC bot which was integrated with Medusa Framework. It gives plugin writers very simple and powerful interface with user database, ACL, m17n, i18n, cron job, session handling, reloadable plugin, persistent database, XMLRPC server.
Yogurt is a Python-based multilayered bot framework for IRC/Instant Messenger. It's a descendant of SugarCube but redesigned at all. It will have Web-based configuration/management interface and provide various extensible and stable Plugin/Filter APIs.