PC Apps
This applet is similar to galaxy repel above but here the stars are repelled by foreground object outlines rather than by motion. It allows you to herd and concentrate stars in a way that you cannot with the motion detection method. However it is a little more sensitive to lighting and contrast in the webcam image. You can just play with the image or read more for advanced controls.
Instructions are the same as invaders above. Hold a diffuse light source (such as a mobile phone screen displaying a bright white image) in front of the camera to grab the projectile. Push it towards the screen to shoot. The light source needs to be somewhat brighter than background light levels so darken the room. It takes some practice to operate. Here's a video of it being used.