Public Presentation Slideshow

I am looking for an inexpensive way to create a presentation style slideshow that I can play on a publicly viewed television at my company.  I would like to see options for both Windows and Mac as I may go with a Mac mini computer.  In Windows I can install the free powerpoint viewer to run a powerpoint slideshow but I am not familiar enough with Mac to know whether or not there is a free or inexpensive option.  I will entertain all ideas so I don't need to stick with Powerpoint.  My goal is to have a system setup that will display a slideshow that can easily be updated by remotely connecting and saving the new content on the computer without having to buy expensive software.