We use a software from our phone system (Intertel) called Oaisys. It works okay, but is somewhat lacking. We tried to replace it with
Outlook Messenger, but it didn't work well and there was a small licensing fee.
We just discovered
Oovoo and while its not restriced to LAN operability, its pretty slick and even allows you to use a web cam. We're going to use this to assist with handling meetings for our staff who telecommutes. It's free and from our early testing, will pay dividends in conducting meetings for those not in the office.