While this is more Techie, than halloweenie, the illusion is kinda goofy... but fun.

iPad2 Halloween Costume- Gaping hole in torso

The man behind the costume is Mark Rober, who works at NASA's Jet Propulsion Laboratory. As he explains in the below video, all you need is two iPad 2s running Facetime video chat, some fake blood and a shirt you don't mind ripping a gaping hole into.

http://www.youtube.com/watch?v=V6p5mbp_M98