IrssiBot is an IRC bot written in pure Java. It has support for being on multiple IRC networks at once, as well as almost unlimited extendability via a very flexible module API. Compiled Java classes implementing this can be loaded and unloaded at run-time. Due to its multithread model, the memory/CPU strain is very low, and it was written with the end user in mind, and is therefore very easy to install and configure. Configuration and the user files are in XML.
