Browser-based IM/VoIP/Video Call application

A client needs a browser-based application (Flash/Java/???) where users can register to communicate with other users. "Communication" can either be * Instant Messaging ("private chat") * VoIP ("phone call") * Video Call Also, there is one function, that is not required, but would be appreciated, which is screen sharing. The application has to open a peer-to-peer connection to the other user, "similar to Skype's behaviour".