Google Talk for Android allowed Google users to send instant messages to one other. Replaced by Google+ Hangouts in 2013.
In May, 2013, it was announced that Google Talk (as well as Google+ Messenger) would be consolidated into Google+ Hangouts. See google-plus-hangouts.
Google Talk for Android allowed Google users to send instant messages to one other. This application replicated many of the features when launching the chat client in a browser via Gmail.
It was available on many other mobile platforms. Quoting Wikipedia:
Google Talk mobile clients are also available for the Palm Pre, BlackBerry, iPhone, Android-based devices and is preloaded on the Nokia N900 smartphone. With the release of the Google Talk gadget, users of all platforms supported by Adobe Flash Player can also use Google Talk. Many other XMPP clients are compatible with Google Talk, and support a variety of other platforms.
Google Talk for non-mobile users: http://www.google.com/talk/