OpenFire is good, robust, free, open-source, under active development, and secure. It is also enterprise size and far more then a small company needs. It does support voice chat and stuff, and is cool, but you have to have a central server.
IPMessenger is small, fast, powerful, secure, and runs on Windows, Mac, Linux, even Android. It is, however, server-less and would die trying to run with a lot of chatters because of it. Further, the programmers are Japanese, and the whole thing assumes you know a bit about IP's and SUBNETs.
Borgchat is from a security standpoint, a program scares the crap out of me. It is not open source, and it has not had an update in four years. After all, it is far harder to hit a moving target. So, Borgchat would be very risky if any computer on your subnet accessed the Internet, ever, in my opinion. (I do security for a living, this seems very sneaky to me)
Pidgin with Zeroconf works, it is cross platform and open source, and does many protocols. It was my first thought, but without plugins all of your information is easily snooped internally. If you go that route, look at any of the encryption plugins. Otherwise, realize you are passing notes that can be picked up and read.
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Similar question: http://superuser.com/questions/37654/lan-messenger-for-windows-with-features-like-google-talk
– jweede – 2009-09-08T13:23:34.7631This is definitely a duplicate. I've seen two other questions (one in the last 24 hours) that are the same question. – ephilip – 2009-09-08T13:35:27.303
you're right, that is definitely a similar question. however, please note that I did look and could not find those questions with the search terms I was using. if you listen to the SO podcast, you'll find that having multiple formulations of the same question is an expected result as sometimes the different text of my question will lead others to the answer in the future if asked in the same way :-) – Joel Martinez – 2009-09-08T15:40:12.950