Pidgin is a GTK2-based instant messenger application. It supports multiple protocols via modules, including AIM, ICQ, Yahoo!, MSN, Jabber, IRC, Napster, Gadu-Gadu and Zephyr. It has many common features found in other clients, as well as many unique features. Pidgin is not endorsed by or affiliated with AOL TimeWarner, Microsoft, or Yahoo.
Gale is an open-source messaging system. It provides private messaging and public chat, uses strong end-to-end cryptography for security, and supports a distributed server network with a range of clients (both console and GUI) available. Gale is currently in early alpha testing. See the homepage for the details of its current status.
srvx is a services package written in C from the ground up, for IRC networks that are based on IRCu 2.10.*. It is both highly efficient and extensible. Its features include Wingate, SOCKS4, and SOCKS5 proxy detection, and all the standard services users expect (ChanServ, NickServ, OpServ, Global).
geektalkd is a simple chat server designed for use with thin clients (like telnet, netcat, or one of the avaliable Perl clients). It features multiple channels, private whispers, and everything else you'd expect from a chat server. Additionally, all of the parser code can be rewritten and reread, so you can install new functions on the fly without restarting the server or kicking users off.
Pipo-BBS allows you to install a BBS server and clients. You can chat via a nice interface with an unlimited number of people (as with IRC). This software is in a pre-release state; it is still very alpha, and can be difficult to even compile. It works under linux, MkLinux, SGI, HP-UX, and Solaris. The only interface for this BBS is telnet for the moment; an interface using GTK has been started. It has been translated into French, English, Spanish and German.
gnuBot is an IRC bot written in Perl, designed to be as flexible and easy to use as possible. Dynamically loadable modules are used for everything from networking to configuration, allowing parts to be swapped out at will. It is intended to be both a channel service bot, like an eggdrop, but also have infobot-like features.