iCU

iCU is a desktop
IN-AND-OUT board, providing an instant reference of who's in or out of your
organisation. Ideal for businesses that operate with a LAN, iCU tells you
where your colleagues are, without you having to leave your desk.

While running iCU, any user on your network simply chooses their status
from a list including "Available", "Away from desk", "In a meeting", or
even "Do Not Disturb"! The user display is updated regularly, so you'll
always know the status your colleagues. You can even see who isn't running
iCU (providing they have run it once before). iCU even sits in the system
tray, so it doesn't clutter up the task bar.

iCU version 2, also allows you to add your own statuses, so you can really
customise iCU to your company requirements.