Google Talk
Google Talk - Google's approach to instant communications.
The Google Talk downloadable application for Windows offers many features for communicating with your friends and family.
Instant messaging — Chat with all of your Google Talk and Gmail contacts in real time.
Free PC-to-PC voice chat — You can talk to anyone else for free who's online and has the Google Talk client.
Send and receive voicemails— If the person you're calling isn't available, you can leave a voicemail.
For more information, visit,
http://www.google.com/talk/about.html