Many times, you receive some interesting video link from your friend while chatting with him/her using Gmail chat. You have to open that link in new tab to watch the video and probably you may forget your friend with whom you were chatting. Jessan Hutchison-Quillian, Software Engineer at Google has the same problem and so came up with interesting feature to Gmail talk.
This feature lets you to preview YouTube and Google Video in chat window itself. If you receive (or send) a link to a video in a chat message, you'll see a preview of the video right in your chat window.
Click the preview, and the video will play right there. Just remember to say something every once in a while or your friends will probably catch on that you're enjoying the dramatic chipmunk more than their conversation ;)
Related : To-Do list for Gmail