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.
icqlib is the most feature complete, open source, library implementation of Mirabilis' ICQ protocol available on the Internet. It currently supports most of the ICQ UDP v5 and ICQ TCP v2 protocols, including new UIN registration, chat, and file transfer. It was written in C for portability, and supports multiple simultaneous connections to ICQ servers, SOCKS5 firewalls for both UDP and TCP protocols, an event-driven, non-blocking interface, and the Russian language (automatically performs conversion from Windows 1251 encoding to Unix koi8 encoding).
climm (CLI-based Multi-Messenger) is a portable, small, yet powerful console-based ICQ and XMPP client. It supports password changing, auto-away, creation of new accounts, and other features that makes it a very complete yet simple client. It is able to send and receive messages from the ICQ network, list users that are offline and online (including their online status), search for users by email address, and look up basic (name, UIN, email) information. climm was formerly known as mICQ.