Cross-platform VOIP + IM client and Linux server

Right now, a group of our employees is using Skype for group chats and conference calls. They're not using telephone numbers, just Skype user accounts.

I'd like to set up a server that will provide similar functionality that is only accessible inside the LAN.

What I'm not looking for is something to use with real telephones.

The main problem with Skype is that the Linux client is terrible and getting worse every year they don't update it. I also don't like the idea of our company data going through Skype servers (yes, I know it's encrypted).

SIP seems to be quite a pain, but there's also GnuGk and Jabber+Jingle.

Whatever it is, I'd like to also support Windows clients.

Is there anything out there that's open source and easy to administer?

openfire (server) spark (client)

both are released under the gpl

You should take a look at Unison (www.unison.com) -- Linux on the server side and Windows/Linux on the desktop.

Although originally stemming from the computer gaming scene, TeamSpeak has become a fairly usable cross-platform VoIP client.

The only problem would be that TeamSpeak in itself does not really offer messaging capabilities. And, as I just found out while checking up again, they are actually charging license fees right now for commercial server usage.