I saw this on science channel, the army will probably start using it in 2013.
Anyway, it was a high-tech laser that was being worked. You would plant it somewhere and it would shoot an invisible llaser in the direction that the player points it at. The cool thing is, it can melt through stuff.
advantages
-invisible to the eye, so a careless enemy could just walk right into it.
-can go through walls so it could stop those campers easy
disadvantages
You could screw yourself over by accidently walking into it.