If there's one criticism we've always had about laser pointers it's that only one person can run the risk of blindness at a time; thank goodness for tinkerer Matthew Leone's DIY Laser Ball project, which opens the door to up to 14 lasers. Leone's awesome Instructable packs a tennis ball with lasers and a microcontroller to create what's described as "a cat-toy for humans" or a remote control "first-generation lightsaber training droid."
Basically, the tennis ball - or any other ball, object or lump you may have lying around - is studded with lasers, each individually controlled by the specially programmed Teensy microcontroller (though you could use our old friend the Arduino too). That responds to a remote control, cycling through various patterns of laser light show.
Altogether, the components come to $78 or thereabouts, which makes the Laser Ball cheaper than many laser pointers you could buy in stores (albeit likely to cause somewhat more confusing during PowerPoint presentations). For something you can build in an afternoon, that's pretty impressive.