Floaters

I thought these were just in my imagination until I saw them in an episode of Family Guy. Apparently, most people see them. Wikipedia:

Floaters are deposits of various size, shape, consistency, refractive index, and motility within the eye's vitreous humour, which is normally transparent.They may appear as spots, threads, or fragments of cobwebs, which float slowly before the sufferer's eyes. Since these objects exist within the eye itself, they are not optical illusions but are entoptic phenomena.

One specific type of floater is either called Muscae volitantes (from the Latin, meaning 'flying flies'), or mouches volantes (from the French), and consist of small spots. These are present in most people's eyes and are attributed to minute remnants of embryonic structures in the vitreous humour.

via