'Space Invaders' is an unofficial street art project whereby small ceramic tiles are anonymously cemented to public buildings, displaying the pixellated villains of the 1978 video game. It's not so much graffiti as guerrilla art, and the best examples blend into their surroundings.
Several cities around the world feature so many examples that maps to 'landing sites' have been produced, but they've infiltrated further than even the project's global home site seems aware; this one occupies a very prominent location in Prague.

Click on the Invader for a closer look.