Intranet Messaging [closed]

"We use Spark at my work.

From the website:

Spark is an Open Source, cross-platform IM client optimized for businesses and organizations. It features built-in support for group chat, telephony integration, and strong security. It also offers a great end-user experience with features like in-line spell checking, group chat room bookmarks, and tabbed conversations.
Combined with the Openfire server, Spark is the easiest and best alternative to using un-secure public IM networks."

"There is Pichat
From the FAQ notes,

Pichat is an own chat system. There are a couple of chat command that are similar to IRC (Inter Relay Chat), but under the hood runs an own protocol and a peer-to-peer chat system that does not rely on a centralised server. Basically you can run an independent chat service and adjust it to your own needs.
After downloading and starting the Pichat software you already have a running LAN messenger, chat server and web server. To make the chat accessible from the Internet you need to configure your firewall, router or cable modem correspondingly. Pichat uses the following ports: 9009/TCP and 80/TCP (in default settings).

You might also find this Stackoverflow question interesting."

At our small business we use BigAnt. It still has a server, but that is fairly light and can be run on a normal machine.

Netsend has disappeared under Windows 7 so you might have to use msg.exe or dmessage ...