Best IM or chat over small LAN?

ICQ has an Intranet only version which works great in a corporate environment. I initially deployed and tested it in the Arthur Anderson network and it was accepted by corporate and is now used by the company globally.

Here is another one that looks good though. http://www.rolesoft.com/english/

hi there , i hope this those links help u .

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Akeni Web Chat :
Akeni Web Chat is an IM system with its browser based client there is no need to install any client side software. It is also firewall friendly and can be used whenever you have access to a web browser. The rich client supports chat, notification and conferencing. This product can be deployed as a stand alone HTTP server
http://www.instantmessagingserver.com/en/home/home.php
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AltME :
AltME - The Alternative Messaging Environment is a communication system with private chat rooms, messaging forums, and instant applications like checklists & calendars. It downloads and runs on your own PC where you are the master with full control over users. Security is provided through encrypted data transfer and authenticated user logon. AltME has a built-in update feature so you are always running the most current version
http://www.altme.com/
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Chatjet :
Chatjet is a flexible chat server system. The client frontend is based on HTML/HTTP, Java or plugins are not required. The server backend is based on a template system that allows to create any kind of chat room - sales chats, moderated event chats, customer support, help desks and more. Chatjet easily can be integrated into existing websites and other server software. A wide range of database systems and interfaces are supported. The server backend includes its own optimized HTTP server and runs on both Windows and Linux (x86).
http://www.chatjet.com/
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ChatSpace :
ChatSpace Community Server is a real-time communication tool. It contains an array of real-time community and support applications including chat, live events, queued help and instant messaging.

www.akiva.com
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IRCXpro :
IRCXpro is an Internet Relay Chat (IRCX) Server that allows IRC clients such as mIRC to connect to your computer as an IRC based chat server. IRCXpro is rich in features such as server-to-server linking, remote administration, message content filtering, auditing, transcript logging, Client Connection Classes, IP Masking & Ident Lookups, News Flash Broadcasting and more
www.ircxpro.com
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123 Flash Chat Server :
123 Flash Chat server software will allow you to build your own chat room with Java used for server mode and Flash for the client. You can build your real-time chat room to put on your website, in forums, intranet and internet portals. 123 Flash Chat allows you to organize chat-events, collaborative work sessions or online meetings. You can also monitor chat sessions invisibly to check for suspicious users. Windows NT/2000/XP are supported.

www.123flashchat.com
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DBabble :
DBabble is a Chat and Discussion Server, which allows users to send instant messages, have private conversations, and create and participate in private or public chat rooms and discussions. Users communicate with the server using either a web browser, or a client for Windows 95/98/ME/NT/2000/XP. The server can optionally be set up so that users can also interact with other users on remote servers, and participate in remote chat rooms and discussions. Discussions can optionally be linked to external usenet (NNTP) groups.
www.netwinsite.com
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Pino 3-Server :
PINO is a TCP/IP based client/server chat system for Windows. PINO 3 is the client application. PINO 3-Server is the central server software managing the individual user connections, verifying user authorizations and ensuring general security.

www.alcomp.net
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Esvon Java Chat :
Esvon Java Chat helps you to connect with your website visitors quickly, easily and inexpensively. You can: create your own chat community: it will make your visitors get back to your web site and chat service repeatedly; moreover attract new ones; organize live conferences and other online events in real time; achieve the highest standards of services, assissting your customers via live pre-sales or consulting support
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Mohammed.A.A
MCSE,CCNA

Have you tried netmeeting  its free and already install on Windows computers -works great for chat -voice -program sharing.  We us it  a lot for training and meetings.

NOTE FOR WINDOWS 2000 USERS: Windows 2000 comes with Netmeeting already installed. You can find it in the Programs group of the Windows Start Menu.

NOTE FOR WINDOWS XP USERS:  If you are using Windows XP, the version of NetMeeting that comes with the camera software is incompatible with Windows XP. However, a compatible version of NetMeeting is available within the Windows XP operating system itself. If you want to use NetMeeting, this is how to activate it in XP.

Click Start, then Search.
Make sure you are looking in all files and folders.
Search for "conf.exe".
Double-click on the one found in the Program Files folder; NetMeeting should open.