Chat in Google Mail recognises Google Video and YouTube URLs as media and automatically converts them to video inside your chat window.
This means you can use chat to send your friend a link to the video, 'Chat In The Name Of Love' (performed by a cast of Google Mail engineers). In addition to the link, http://www.youtube.com/watch?v=ZVuXmmXOIj0, they'll see the video itself and can press the play button to watch it inside the chat window.
Note: Chat in iGoogle and orkut does not support inline media. Users chatting through those products will only receive the Google Video or YouTube URL.