The Abbott's Sphinx Moth (Sphecodina abbottii) caterpillar, have discovered that best way to [hopefully] survive their harsher younger years is to have a big ol' eye spot on their butts. This isn't just your average eye spot, though. I mean this thing looks incredibly real - right down to the white reflection spot in the "pupil." If you poke the caterpillar, the little guy will squeak and try and bite the attacker.
This is the velella (Velella Velella), a small free floating hydrozoan. It's currently the only known species in the genus. They're also known as sea-rafts or by-the-wind-sailors, for the obvious reason that it uses the the "sail" you can see in this image for locomotion. Because of this, they are often found washed up on beaches.