Google Talk allows us to connect to others IM networks, but with a drawback, Google Talk doesn't include a place to configure transports (i.e. the ability to provide a username and password for the IM network of choice). So one has to install another Jabber client in order to configure that transports.
After that you can use Google Talk to talk to all of your IM network friends.
For more information see:
Jabber Transports (or How to Connect Google Talk with Any IM)