www.thesentryproject.com
This is pretty neat... It's an Airsoft/paintball/anything program that you can build yourself using a few servos or a kit that you can buy from them.
I don't think it would be very practical to use in games though, mostly because it still requires a laptop to run, which could be stolen if the system was left un-attended (which is the whole point of the sentry...) and also people may not call themselves out. There are a variety of other problems as well (wind moving brush/plants etc) but I thought it was a cool find. Enjoy.