YSM (You Sick Me) v7 is an ICQ console client. It was originally meant to run in Linux, but it has been successfully tested in Linux, BSD, Win32, BeOS, and QNX. The client requires no extra libraries, and is written in the C language. YSM is based on the last ICQ protocol version, v7/8.
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).
Twisted Words provides implementations of a handful of IM protocols, including IRC, MSNP8, OSCAR, TOC, and Jabber. It also comes with a multi-protocol server built around a few simple interfaces, the goal of which is to facilitate implementations of novel servers, clients, and bots. Out of the box, it comes with server that accepts connections over IRC and PB and seeks out installed third-party plugins to support other interfaces (for example, a Nevow LivePage-based Web interface).
daim is an IM client and library that supports many features, including add-ins, AIM proxy, buddy icons, chat, direct IM, file transfer, file sharing, send buddy list, and Trillian secureIM. It also supports retrieving News and Stocks, and scripting through Apache BSF. It also supports ICQ, can emulate Apple's iChat client, and supports iChat available messages as well.