IMO, there are 3 types of solutions categorized into: hardware- and software-based, and hybrids.
Hardware-based are –
Montage, Spyder, Hippotiser, etc. Expensive if purchased. Rental is cheaper depending on duration of usage.
Software-base are –
Dataton Watchout, AvStumpfl Wings Platinum (both Windows-base), and some Mac-based solutions – ProPresenter from renewedvision.com and Multisreener (freeware) from zachpoff.com
All are basically based on a Master PC with a master timeline controlling the various nodes (slave/display PCs). Licences come in the form of USB dongles, one for each node including the Master. So generally, for a 3-screen presentation, you would need 4 licences and 4 PCs/Macs.
Hybrids –
Basically involves a compact-flash controller controlling other compact-flash or hard-disk players. However, this generally means the controller program must first have been programmed on a software such as those earlier mentioned, and tested before downloading to the players. In other cases, the first player in the chain can be the controller of the others down the chain, therefore not requirng a separate Master controller. Some work akin to digital signage solutions with a playlist.
However, framesync between players is questionable if this feature is required. The hardware- and software-based solutions earlier mentioned all do framesync between the output displays/slaves.
Thomas Leong