I've used a few different ones--though not with Artemis per-se.
If your Comms stations have dual-screens: one for Artemis, one for the communication app... That might be awesome.
Google Hangout: PRO: allows multiple people to join, cross platform. CON: uses a good bit of CPU power, audio quality is a bit lacking if you're not wearing a headset. Needs Google ID and Internet bandwidth.
Skype: PRO: Audio quality is awesome. If you ensure you ONLY use audio, all traffic in a multi-party call should be peer-to-peer and not require Internet bandwidth. CON: If someone turns on their webcam, the handoff from audio to video can destabilize things a bit.
TeamSpeak: PRO: pretty lightweight when I last used it; LAN hostable. Not sure how Mac joins though.
Now, if you don't mind going the hardware route...
Cell phone: Seriously. Just get a conference call up and running and wear your headset. Done.
DIY Intercom: if you really want to go deep geek, see this:
http://www.epanorama.net/documents/telecom/telephone_intercom.html : scroll down to the "Cheap Intercom" circuit item from
ben@staff.prodigy.com. Use wireless phones.
OK, probably too much information. Have fun!