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.
Jackson Pollock — move the cursor, splatter paint in real Pollock palette. Click to change colour. Quietly online for two decades and still hypnotic.
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.