[pja]

IIRC the really small white ones that you see when looking up at a blue sky are actually white blood cells in the arteries on the inner surface of your eyeball.

[heed]

It's called 'blue field entoptic phenomenon.'

http://en.wikipedia.org/wiki/Blue_field_entoptic_phenomenon

[iceron]

Yes, Blue field entoptic phenomenon is the name.

I've also heard of visual snow, which seems to be related to migraines.

http://en.wikipedia.org/wiki/Blue_field_entoptic_phenomenon

http://f1000.com/posters/browse/summary/1090714