Mostly what you are seeing on the top are EOTech holographic sights. They're pretty cool. My friend has one on his AR-15. There is a little window you look through that displays a circle with crosshairs. You can keep both eyes open when doing this (good for being aware of your surroundings, etc). Even if the device gets muddy, cracked, half destroyed, it will still display the reticle in whatever small area is not affected. Other items are Trijicon scopes. Again, these have lighted crosshairs or dots, but magnify the image (EOTechs don't magnify anything, just make target acquisition quicker.). Do some Google searches on those two names and you'll see plenty of pics.