Cross-platform LAN messenger that can run on multiple user accounts of the same machine at once?

Question

I'm looking for a (free) program that allows me to chat to other computers on my network (over LAN).

Desired features:

• Internet connection not required
• Available for Windows 7 and Mac OS X 10.6.8 (Snow Leopard)
• Hopefully good looking UI, preferably with multiple/customizable themes, but not necessary
• Preferably File transfer
• Text chat (of course), and maybe voice chat too (but not required)
• It should be able to be run on multiple user accounts on the same computer at the same time (ie.: no port conflicts - like in Qualia LAN Messenger - but if anybody knows how to bypass this, please tell me)

Answer 1

As described in my answers to the questions Software to send files through my private network and Simple serverless messenger for Linux, you could use any XMPP client that supports Zeroconf (Apple’s implementation is called Bonjour).

• No need for all participants to use the same client.
• No XMPP server (whether online or offline) required.
• Clients find each other automatically.
• Text/voice/video chat and file transfers possible (all these are client features, so it depends on the clients involved).

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List of XMPP clients: http://xmpp.org/xmpp-software/clients/

Jitsi would be an example for a cross-platform client; however, it seems that the Zeroconf feature was removed and they intend to reimplement it in a better way; I’m not sure if this is done yet. You could, however, use SIP with Jitsi. Here's how to set it up. As far as I know, the only difference between SIP and XMPP with Pidgin (details below) is that with SIP, you need to know the internal IP address of any computers you want to chat with. This shouldn't be too much of a problem because you can use an IP scanner tool, like Advanced IP Scanner for free.

Pidgin is another one (note that they recommend to use Adium on Mac OS).
When going with Pidgin (+ Adium):

• Windows users of Pidgin have to install Apple’s Bonjour Print Services for Windows.
• After that they can add a local XMPP client with New Account → Bonjour (thanks to shub for this info).

Features that Pidgin + Bonjour give that you asked for:

• Free
• Mac + Windows
• No port conflicts
• File transfer
• Not bad-looking

Answer 2

Softros LAN messenger supports both the Windows and Mac OS X platforms.

Pros:

• An Internet connection is not required.
• A dedicated server or server-client is not required.
• It supports Terminal Services (great for large networks).
• All messages are encrypted by secure algorithm.
• It sends messages and files. No port conflicts.

Cons:

• Application is not free. It has 30-days trial without functional limitations.
• Voice chat is not supported.

Answer 3

For ~ $20.00 USD you could install OS X Server which has a built in XMPP server. Then you could use any XMPP client, like OS X Chat.app, or any of the chat clients listed here.

If you don't like the $20 option, you could look here for a free xmpp server.

Another option would be to purchase/repurpose a Raspberry Pi. Install a simple XMPP server on it. That way you don't have to keep an expensive laptop/desktop running all the time.