When someone sneezes, what do you say?

Scat there! — Mom and Grandma used to say this. Thinking about it more, it’s almost a scaring away of the thing that is making you sneeze or maybe the germs of your cold. (??) Possibly to ward off the fariries tickling your nose. who knows.

Bless you. — This one is very church-worthy. A general blessing for either better fortune, a cleaner environment, peace from above, whatever it is that might make you stop sneezing all over everyone. :]
Here’s what Snopes found.

Gesundheit! — Literally “good health”, this one has always been popular. For some reason, German exclamations seem to be popular, but not regular german words in general.

À tes souhaits — “To your wishes” or “May your wishes(dreams) come true” this is used commonly in francophone culture, and if one knows the sneezer very well, it’s À tes amours, “To your loves”. I suppose a wish for your wishes is much akin to wishing you well.