Very sad news. I can't think of anyone who was more of an influence on what scared the bejeezus out of me when I was growing up. Giger's art has always had a very special place in my heart, mainly from my first encounter with monster(s) in the Alien franchise.
Unique, bizarre, and terrifying. His character designs for those films (games, comics, etc.) will always be some of the scariest visions of extraterrestrial life out there. A veritable worst case scenario of encountering life outside of our solar system. His other work is memorable and shares a common voice, equally as tormenting as his additions to film. I always knew I was looking at a Giger piece, and I always loved seeing what tortured visions his brain conjured.
We lost a great mind today.