@Steve, the app you're thinking of is likely
http://www.cinesync.com/
It's BRILLIANT software that allows people all over the planet to view, markup, play, stop, rewind HD [or any frame size] footage in perfect sync with one another!
The only pre-requisite is that ALL parties have ALL the same source footage on their systems BEFORE the Cinesync session begins.
I've used it extensively with producers and colleagues in two different countries.
W