Quassel IRC is a modern, cross-platform, distributed IRC client, meaning that one or more clients can attach to and detach from a central core, much like the popular combination of screen and a text-based IRC client, but graphical. In addition to this unique feature, it aims to be a comfortable chatting program.
Based on ircu, Nefarious has features from various IRCds (Asuka, Bahamut, UnrealIRCd, and UltimateIRCd) with the intent of adding needed features to ircu without becoming too bloated or subject to anti-user behaviors. Most features can be activated or deactivated in the config file. Nefarious includes optional support for RBL-based blocking or marking, fakehosts, accounts, login-on-connect, half-ops, ban exceptions, mIRC color filtering chan-mode, network-wide bans (g-lines), and much more.
envbot is an advanced modular IRC bot coded in bash. Features include SSL, IPv6, modularity (with several good modules already), transport modules (including OpenSSL, GnuTLS, netcat, etc.), loading/unloading/reloading of modules at runtime, rehashing configuration at runtime, advanced access control, and much more.