Magnetic Accelerator Kits
The Magnetic Accelerator Kit for Think Geek uses magnets to launch stainless ball bearing at highspeed. See the video for how it works.
Source: ThinkGeek
The NeoCube Alpha Magnets
This 216 Rare-Earth Magnet Balls can transform into many shapes for your fun. Will cost you $30 $40 (Thanks, Sasha). See video for how you can play with it.
Source: The Neo Cube