This is a copy and paste from Now Watching website (too lazy to put them in my own words):

Now Watching is a software that lets you share what you are watching on Youtube and other video websites with people on your MSN Messenger. The title of video you are watching is displayed on your MSN Messenger's now playing box.

More info on:
tokback.com/nowwatching/index.htm

!!!
it´s really really great to share youtube videos on messenger. really a great thing that was incluede on the lastest versions of MSN.

Thanks for your messages！