I'm looking for a free/open source Instant Messenger solution targeted at LANs. Essentially, I'm looking for a replacement (and more advanced) system for WinPopUp/LinPopUp. Just something to let the people in a small business environment communicate. Being open source is not an absolute requirement, but free is. The other big requirement is the availablity of clients for both Windows and Linux platforms.

I'd been thinking of writing something of my own under Java, but implementing a GUI and Sockets in the same application is a tad above me right now.

Look at How to setup a Jabber server.
You can use any jabber-capable client with this, like GAIM.

Cool, I'll give that a try. Thanks so much!