irspider is a Linux-based IRC smartbot. It is very similar in functionality to a search engine on the Web. irspider goes out and spiders DCC offer bots that are in the same channel as itself, and stores tons of information on what those particular bots are offering. Users are then able to query irspider for matches against its database, or request a masslisting of everything that it knows about.
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.
KVirc is a Visual Internet Relay Chat Client based on the Qt library. Features include a MDI user interface, drag & drop, built-in scripting language (aliases, popups and events), customizable backgrounds and colours, DCC (chat, send with resume, voice), CTCP functions, mIRC colour support with preview, notify list, online help and configurable logging.