YouTube Social is web service for watching videos together with friends, synced in real-time, with chat thrown in for good measure.
And it's easy to start with: you take the URL of a YouTube video, say: http://www.youtube.com/watch?v=6G5JaicYuVU, and add the word "social" after "youtube," like: http://www.youtubesocial.com/watch?v=6G5JaicYuVU.
You then get a rich interface with your video taking center stage, and a TinyURL link that you can send to your friends. They just need to click the link to join you. Now, whenever you play the video, pause it, or even scrub to a different position, their videos follow suit. There's a chat pane where you can all talk about what you're watching, too. You can also give the "remote" to someone else so that they get to control the video.
More Info: YouTube Social