Chat application by Google. Check out what Google Talk can do:
Instant messages: share quick thoughts in real time
Status updates: see if friends are around and what they are up to
File transfer: instantly send and receive files, pictures, and more
Free long distance: make PC-to-PC calls to people anywhere in the world
Audio conferencing: talk to multiple people at once
Gmail integration: chat with your friends on Gmail
You can also get the video chat plug-in to make voice and video calls within Gmail, iGoogle and orkut.