Google Talk (GTalk) is a free Windows and web-based application for instant messaging and voice over internet protocol (VOIP). It lets you send instant messages, do automatic file transfers, check friends' status updates, make PC-to-PC calls, and make conference calls to multiple people at once.
Additionally, you can install Google's video chat plugin to make voice and video calls within Gmail, iGoogle and orkut. http://alternativeto.net/software/google-voice-and-video-chat/
