Skype - cross platform telephony
I was just chatting with Joshua Archer and we tried out Skype. It looks really promising, but we didn't get to have an actual conversation over it because I didn't have headphones, and the feedback from my powerbook's internal microphone was pretty nasty.
Assuming the addition of headphones clears that up, this should be a great addition to our suite of collaboration tools (and would wind up getting a spot in our potential NMC 2005 session in June...)