Gmail has a built-in Google Chat gadget that lets you send instant messages to people in domain, directly from your email application. 

Send instant messages
Send instant messages
Quick steps on using Google Chat to send instant messages and use voice and video chat.
Google Chat Help Center
Find out more about using IM, voice, and video chat with the Google Talk gadget in Gmail.
Download the Google Talk client 
Download the standalone Google Talk client to chat via IM, voice, or video with users outside your domain,