I'm always impressed by the things anime fans manage to create, be it music, art, costumes, or anything else. However, there are some works that really blow me out of the water with just how amazingly well-done and creative they are; among those, here are a few of my personal favorites:
Attack on Titan: a spring-loaded 3D maneuver gear.
Gurren Lagann: the construction of a heavens-piercing drill.
(formerly synced up to the grand finale of the series)
Touhou Project: a short anime series based on the games made entirely by fans.
Jojo's Bizarre Adventure: a complete English version of the second opening.