[stephen_g]

Wow, I get this, but very minor (the bits are mostly transparent and really small, but I can see them looking at the sky). I didn't know for sure whether they were physical objects or something in my mind, but they way they moved made me think they must be physical. Good to know that's actually a thing!

[dchichkov]

These are white blood cells moving in the capillaries in front of the retina of the eye.

[norswap]

Those are simply nerves that run in the eye, everyone has them.

[stan_rogers]

They're often just very small dust particles and minor variations in the thickness of the fluid coating the cornea as well. Those would be the ones that seem to float gently downward (sometimes almost forcing you to try to follow them, which is, of course, impossible) but which tend to reestablish their position after every blink. Annoying, but that's all.