We have one of these in our Synagogue (based in the UK) and it was cheap and easy to set up. Here's some details on how we did it...
We have a large flat-screen TV (that can also operate as a computer monitor) on the wall. This was donated. On the other side of the wall is a regular, old Windows XP PC, with internet access. The PC runs internet explorer in full screen (kiosk mode) permanently looking at our own special website.
The website is built on Joomla, which is an open source content management system. The TV in the synagogue displays the front-end (public view) and we can use the administration view to edit the content from any internet connected computer.
We wrote some really simple Joomla components to display display static images (GIFs) and Flash Animations. When you upload each file to the system you can specify how long to display it for, dates for which it is valid etc.
We have ours in the foyer and it is primarily used to advertise events / announcements etc.