Syncarella is an online video collaboration tool that allows up to 4 users to watch and markup video simultaneously across the web in realtime.
It's weblaunched which means theres no install.
It runs on any platform that has Java 1.5+
and best of all it's free for single-users, free to join a session, and free for 30 days if you want to start your own sessions.
Give it a spin, and let me know your thoughts.