Dichroic and trichroic prisms have interesting qualities that make them useful in certain kinds of image capture systems. They are also cool to play around with in the dark.
Here's another nice demo:
This one even includes the unboxing:
If you'd like your own, I used this set of three to nice effect in a screen test on a recent film project, but I for various reasons I ended up buying the standard triangular prisms that look like Toblerone packaging or the Pink Floyd cover art.
Image: YouTube / sooda