"Hello" (videochat) is pretty neat, imho. You can share tabs, video/voice and chat. It's great for training users. All you have to do is send an URL, and it works in Chrome too.

It's based on HTML5, but because of how webrtc has to punch through firewalls, it uses one of their partners to initiate the connection (Telefonica?). After that it's mostly peer to peer.