Society expects someone to answer a sneeze with some form of "bless you"; what is your reaction to someone sneezing?
"I acknowledge the irritation in your nasal mucosa"?
I usually go with gezundheit, proost, salute, or salud: German, Dutch, Italian, or Spanish for "to your (good) health". An option from other languages is prosit: Danish, Swedish, or Norwegian for "may it help"; or будьте здоровы / "bud'te zdorovy": Russian for "be healthy".
As others have suggested, offering them a tissue would be nice.
Otherwise, silence may be best. In Japanese, Chinese, Korean, and Vietnamese cultures, it's quite uncommon to respond at all. The traditional "(god) bless you" after someone sneezes came from the Christian-European folk stigma that the subject was expelling a demon from his/her body, and (s)he should be blessed to prevent it from returning. Wouldn't it be easier to simply let an archaic traditions die than replace them with "the secular options"?
"Bless you" actually originated during the days of the plague. Since sneezing was a sign of infection, "Bless you" was a response acknowledging that the person sneezing would soon be dead. It was seen almost as a curse. Knowing the history of the phrase, there are some people I will say it to.
Generally I stick with, "Gesundheit!"
Wow might be a good thing to say. At least you paid them some due. If you start imagining others nearby saying things like "ass chew" or maybe "is your cousin tight" you know it's time to go have your hearing checked. I've even heard fakers come through with the "F" word in sneezing.
I don't say anything. I think it's weird that we reply to a sneeze, but not a cough, burp, fart etc. When I sneeze in a way that might be disruptive to others, I say "excuse me". This is my personal mission. I wish to change the world in this small way.
i say "bless you" and then i add "these are my OWN blessings, and i have a limited number, so you only get two. if you sneeze a third time you're on your own." the original practice came from the time of the plague (one of the times of one of the plagues) since sneezing was an early symptom and people got all scared for (and of) you if you snoze.
I don't react in any way...except to ask if they need a tissue ..maybe. I also don't say thank you when people say bless you after I sneeze. I ignore them. I don't care what society expects. I secretly hope they ask me why in regards to my obvious indifference.