Google Talk

Google Talk is One free application offered by Google Inc. is an application that enables Gmail users to chat between. Provided you have a Gmail account then you can use password and username to login. not just chat by text you also can chat by video and voice. to do this you must download the video and voice plugin for google talk. it's a freeware

this is what google says about this application :
Text chat

• 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

Voice chat

• 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

this applicaion can run under windows and mac  operating system.
website : http://www.google.com/talk/