Google Talk for Mobile

Want to stay in touch with your Google Talk friends and contacts while on the go? Check out Talkonaut. It's a Java-based client that can be added onto your mobile phone, giving you access to your contacts on Google Talk or any Jabber network. . The application will work on pretty much all popular J2ME handsets. You will need a internet connection in your phone(GPRS EDGe or 3G etc) to use the Talkonaut service since it does establish a TCP/SSL connection.

Even though Google Talk supports VoIP, this free Java client only supports instant messages, so no free voice calls yet.
There are other clients available and will be soon added to this post

Check out Talkonaut here