IceChat is an IRC ( Internet Relay Chat) program designed to run on Windows 9x/Me/2k/XP/2003. It is used to connect to IRC Servers and chat in channels with other IRC Users. You can connect IceChat to an unlimited number of servers, all in one session (No need to start multiple copies of IceChat)
IceChat is a freeware Windows IRC client written in Visual Basic. It has an easy to use interface. It includes scripting support, aliases, popup menus, all of which are easily edited within the program.

Link: http://www.icechat.net/site/

Klient is an IRC (Internet Relay Chat) application for Windows. Klient features a modern customizable interface, multiple server support, highly robust scripting, external .dll support, and much more. Klient's interface can be tailored to your liking with features such as fully customizeable toolbars, various window "styles", and advanced display settings.
Klient is also fully Windows XP theme compatible. Klient can connect to as many servers as you want at the same time. They can be from the same or different networks. Klient offers two ways to add scripts. You can create scripts with Klient's built-in script editor in any language that supports Active Scripting, including VBScript, JScript, Perl, and Python. Klient adds 70 objects to its script interpretor to extend the script languages and allow script code to talk to Klient.
For the ultimate in speed and flexibility, Klient can also run scripts contained in external DLL files.
In addition to functionality, DLL scripts can add custom forms to Klient that appear to be part of the program. Because compiled executable code is so much faster than any runtime interpreted language, DLL scripts can run millions of times faster than their interpreted script counterparts!

Link: http://www.klient.com/

Miranda IM is a multi protocol instant messenger client for Windows. Miranda IM uses very little memory and is extremely fast. It requires no installation and can be fitted on a single floppy disc. Its powerful plugin system makes Miranda IM very flexible. Only the most basic features are built in, but there are currently more than 350 free plugins available for download that allows users to extend the functionality of Miranda IM.

Link: http://miranda-im.org/

mIRC is a shareware IRC client for Windows. It is developed and copyrighted by Khaled Mardam-Bey. mIRC is a highly configurable IRC client with all the goodies other clients on UNIX, Macintosh and even on windows offer, combined with a *nice* and clean user interface. mIRC offers full color text lines, DCC File Send and Get capabilities, programmable aliases, a remote commands and events handler, place sensitive popup menu's, a great Switchbar, World Wide Web and sound support, and... a lot more. mIRC is shareware but not crippled in any way... The registration is a one time payment of US$20.00, and licenses you to use future versions of mIRC as well.

Link: http://www.mirc.co.uk/

Trillian is a fully featured, stand-alone, skinnable chat client that supports AIM, ICQ, MSN, Yahoo Messenger, and IRC. It provides capabilities not possible with original network clients, while supporting standard features such as audio chat, file transfers, group chats, chat rooms, buddy icons, multiple simultaneous connections to the same network, server-side contact importing, typing notification, direct connection (AIM), proxy support, encrypted messaging (AIM/ICQ), SMS support, and privacy settings. Without stealing your home page and with no other included software, pop-ups, or spyware, Trillian provides unique functionality such as contact message history, a powerful skinning language, tabbed messaging, global status changes (set all networks away at once), Instant Lookup (automatic Wikipedia integration), contact alerts, an advanced automation system to trigger events based on anything happening in the client, docking, hundreds of emoticons, emotisounds, shell extensions for file transfers, and systray notifications.

Link: http://www.ceruleanstudios.com/