I've been collecting different optical illusions for over a year. Here are some of my favorites. (I've tried to credit the creator and/or sharer whenever possible.)
This is a static image, but these dinosaurs look like they are moving.
Credit: @AkiyoshiKitaoka

Credit: @AkiyoshiKitaoka
Shared by: @veryapetv

Credit: @AliceProverbio

Credit: @waitbutwhy

Shared by: @ThatEricAlper

The green lines and the blue lines in this picture... are actually the same color.

Shared by: @social_brains
Credit: @AkiyoshiKitaoka

Credit: @gsarcone
Shared by: @KRUZAA_
Shared by: @pickover
Credit: @MichaelGalanin
Look at the dancer on the left and then the one in the middle.
Then, look at the dancer on the right and the one in the middle.
Shared by: @SteveStuWill
Credit: Nobuyuki Kayahara
Shared by: @SteveStuWill
Credit: Robbie Khan