Skype.
I’m sure you’ve heard of this. I haven’t used it. But if I try to do some weekend collaborative gaming (“Hey, want to play WoW? Hey want to play ESPN? Hey, I killed your dude.”) then this might be a great alternative than trying to run a Ventrillo server in-house.

Main drawbacks I see:

1. Everyone is on AOL IM; what a crappy product. DirectIM doesn’t work for crap through a firewall; makes no sense. It’s a damn shame because IM’ing pictures with text is so much more involving. In iChat I can drag and drop pictures no problem but it can never connect when you aren’t on a LAN with the other person. If I’m on a LAN, I just walk over and tell them face-to-face. Kinda defeats the purpose. Not to mention you can’t leave messages for people on IM, like ICQ does.
2. You have to pay for voicemail. I know everyone has to make a buck but this kinda sucks. AOL IM is terrible because I can’t leave a message and Skype really could have gotten one up in this respect. It’s different products but I think voicemail-type functionality is really key. Beats checking to see if someone is online and then sending them an email if they aren’t. You have to know two pieces of information for all your contacts; email and screenname. Boo.
3. You can pay to do Computer->Real Phone calls. So money/balance is in the UI. I hate seeing that. What if I call a real phone by accident? Do I get charged? It’s a loaded gun in my mind just like Amazon’s one-click checkout feature. Nice but I’d rather have the safety on.