Best for visual impact.
For pure "how did they do that" reactions:
Akiyoshi Kitaoka's Illusion Gallery — Japanese psychology professor's archive of original optical illusions. Static images that appear to move, even though they don't. His "rotating snakes" pattern made it onto magazine covers and album sleeves.
This Person Does Not Exist — refresh the page for an entirely AI-generated photo of a human face. None of them exist. The unsettling quality has only grown as the technology has improved.
Sketchfab — 3D models you can rotate, zoom, and explore in your browser. The "staff picks" feed alone is a working art gallery.
Falling Falling — colored bands sliding down your screen forever, with a single tone. Hypnotic in ways that shouldn't work but do.
Drawing Garden — anonymous strangers drawing flowers together on a shared public canvas. The collaborative aspect is the cool part; the result is incidental.
For more in this register, the Optical Illusions and Hidden Gems categories are the deep dive.