In an exhibition, I have to synchronize 4 different computers running 4 sections (each 720p) of a long video. The system was built with 4 mac mini, a draft-n router and a free program called Multi-screener (http://www.zachpoff.com/software/multiscreener/) created by Zach Poff.
This program is created by Max/MSP and relies on Java runtime. So it is important to install Max/MSP first and update your JRE. The application is pretty straight forward, just run the MultiScreener server on 1 of the computer within the LAN network, then run MultiScreener clients on other computers, they will automatically sync. It is important to make sure the length of the movie files are the same. You can also change the frame offset using left and right key on your keyboard to fix small glitches on site.