SyncMeTo

Watch YouTube or listen to Soundcloud together online

Reviews
Discussion
Made by Ben WegScheider who does not have Twitter: http://www.benw.de/. I synced with my girlfriend a couple of times and I loved the simplicity, so I decided to hunt it :). Also check out the experimental work by http://random.nu/ I think another interesting feature to point out, that you can share your excitement with everyone by using the screen as a drawing canvas!
Ben has no Twitter, he wanted to comment ;)
I'll admit, it is quite a bit more simple than https://plug.dj