Videmus is a very, very basic in-browser video player with one function: It can synchronise multiple people, providing they have the same video to play.
All it takes is for one person to paste a number to another (this number being the assumed starting time) and their video can instantly syncronise.
It's intended to be an alternative to streaming for social movie watching online - one that is more bandwidth-efficient and allows for pre-downloading the movie file, thus avoiding the usual problems of buffering and the resulting out-of-sync viewers that often plague such activities.
For demonstration purposes, I have prepared a few public-domain movies from archive.org, but you can easily point it to your own. It will accept webm or mp4 format, providing the browser supports it - which usually means webm is the safer bet.
All About Polymorphics (1959), explaining the then-experimental technology of distributed computing and networking.
It's A Wonderful Life (1947), a film initiatially poorly-received that went on to become a staple of Christmas viewing tadition.
Finding His Voice (1929), an animated educational short and one of the earliest produced using sound-on-film technology - which is also the subject of the short.