Clients

Adns
A DNS resolver library for C and C++ programs, and a collection of useful DNS resolver utilities. (C) [GNU/Linux, Unix]
http://www.chiark.greenend.org.uk/~ian/adns/

Amsn
A MSN Messenger clone for Linux. FAQ, software download, translation resource and Spanish site translation. (Tcl/Tk) [Linux, Windows, Macintosh]
http://www.amsn-project.net/

Babylon
A client-server whiteboard/chat program which can be imbedded in WWW documents as an applet, or run as a standalone application. (Java)
http://www.visopsys.org/andy/babylon/

BitlBee
Tunnels instant messaging traffic (including MSN, ICQ, Yahoo, Jabber) to a virtual IRC channel and virtual IRC queries. Documentation, screenshots, bug tracking, and downloads.
http://www.bitlbee.org/

Cervisia
A graphical frontend to CVS. (C++) [GNU/Linux, Unix]
http://cervisia.kde.org

Curl and Libcurl
A tool and a library (usable from many languages) for client-side URL transfers, supporting FTP, FTPS, HTTP, HTTPS, GOPHER, TELNET, DICT, FILE and LDAP. (C) [GNU/Linux, Unix, Windows, VMS, RISC OS, OS/2]
http://curl.haxx.se/

CvsGraph
A utility for generation of graphical representation of revisions and branches from a CVS/RCS repository. (C) [GNU/Linux, Unix]
http://www.akhphd.au.dk/~bertho/cvsgraph/

Dailystrips
A utility to download your favorite online comic strips each day. (Perl)
http://dailystrips.sourceforge.net/

Irssi
Modular realtime chat client supporting IRC and SILC protocols. Supports multiple servers, themes, perl scripting and plugins. (C) [GNU/Linux, Windows]
http://irssi.org/

Lyntin
An extensible Mud client and framework for the creation of autonomous agents, or bots, as well as mudding in general. (Python)
http://lyntin.sourceforge.net/

NetSender
An instant messaging system for Microsoft networks. (Visual Basic) [Windows NT/2000/XP]
http://cvs.uwc.ac.za/Projects/NetSender/NetSender.html

Spicebird
Personal information manager based on Mozilla Thunderbird's code developed by an Indian company called Synovel. It provides e-mail, calendaring and instant messaging and also works as a newsgroup client.
http://www.spicebird.com/