Gmail has a built-in Google Talk Gadget that lets you send instant messages to people in your domain, directly from your email application. You can also chat with people outside of your domain using the stand-alone Google Talk client.
Send Instant Messages
Quick steps on using Google Talk to send instant messages and use voice and video chat.
Google Talk Help Center
Find out more about using IM, voice, and video chat with the Google Talk gadget in Gmail.