Graffiti made from light, done with glowsticks, fireworks, etc. and long exposure photography.
This one's my favourite :-)
It makes me wonder, however, about whether it's really graffiti. Street art, certainly, but the fact that it's:
a) not permanent and
b) not visible on the street, only to the artist/website viewer
makes it (to me at least) a different, and much more exclusive kind of thing.