Google Talk & Chat

Gmail has a built-in chat gadget that lets you send instant messages to people in your domain, directly from your email application. You can also chat with people outside your domain using the standalone Google Talk client.

Please note that Google Talk may not be available depending on your department's policy.

Send instant messages
Send instant messages Use Gmail's embedded chat gadget to send instant messages to colleagues and use voice and video chat. 
Google Talk overview (PDF) Learn to make voice calls and send instant messages outside your domain using the standalone Google Talk client.
Download the Google Talk client Download the standalone Google Talk client to chat via IM, voice, or video with users outside your domain.