http://www.youtube.com/activesharing_about
About Active Sharing
Are you a video trendsetter, an expert at finding the cool stuff on YouTube? Now you can share what you find immediately by Active Sharing with other YouTube users.
Click the "Start Active Sharing" button, then go watch some videos. Your username will show up on the page of the videos as you watch them, and a list of the latest ones you've seen will appear in your profile.
(YouTube respects your privacy. This information only shows to other YouTube users for 30 minutes, so get busy and show off your good taste!)