By far the busiest post on the Sixteen:Nine blog is one that lists the many options out there for free, kinda sorta free, or free to start with digital signage CMS software.
One of the oldest - and among the few that are legitimately free - is Xibo, an open source digital signage solution that started as a student project in the UK many years ago.
It's still around and has grown up and dramatically evolved. Xibo is still open source and still fundamentally free, but a company has developed around it to provide supporting services - things like hosting and technical help. The open source Xibo code in its early days was definitely stuff only propellor-heads could make any sense of and use, but Xibo now has friendly installers and easy user interfaces - making it a product anyone can easily work with.
I spoke by Skype with Dan Garner, the student who first developed Xibo in Brighton, England, back in 2004. He now runs the supporting company, Spring Signage.