If you have Lightroom, that will do it - it's in File > Auto Import settings, you can specify a watched folder and it will update and auto import to LR when a new image appears, once you have it set up, run the slideshow or just put the latest pic full screen and it'll update as they come in - you can also add some develop settings as well as save them as a preset.
A free version is a bit more involved, but you can get the software here
http://www.eventphotographersociety.co.uk/phpBB3/viewtopic.php?f=60&t=160 also think Jalbum has a server mode that will do it (or it's linked in with the above)