If you go with Live Messenger as Russ suggested do not forget to install
Messenger Plus, a free addon, which extends Live Messenger capabilities
(really nice features :)) - I use it on daily basis and I pretty sure it
meets all your requirenments, it also has nice Voice feature, which gives you
opportunity to save some money on phone calls.
Jabber is also a niceoption, but jabber is not communicator, and it was not
accuired by Cisco. Jabber is a protocol based on XMPP. Cisco accuired Jabber
Inc., which was / is the comany to produce its own implelmentation of Jabber
server and Jabber client. There are a few open source jabber server
implementation that mostly run on *nix systems. However to use jabber you do
not need to run your own server, there are several organizations / companies
that will allow you to create a free jabber account (the most known is
http://www.jabber.org) install Jabber clients on employees computers, there
are plenty to choose from.
One more thing to rememeber about jabber is that it can run transports on
the server. Transport allows clients who use jabber to comminucate with
people who uses different IMs for instance yahoo messenger, msn and so on...
So it can be a good option if you plan to communicate also with peole
external to your organization and you do ot want to install 5 different IM
clients on desktps.
To read more vissit: jabber.org
And my five cents, even that I have listed quite a few advanteges of using
jabber, Live Messenger from Microsoft is my first chooice, due to great Voice
feature and stability.
With kind regards
Krystian Zieja
http://www.projectenvision.com