Everyone has already mentioned a lot of superstitions all of which people know about growing up and such, but I think very few people take them seriously. Some people might reference them ironically if they happen though. Like if you accidentally break a mirror someone might joke that you have bad luck now. I think what's more common for Americans to believe in are conspiracy theories. Lots of people have serious beliefs in them, and they're a lot more common place than traditional superstitions. They're pretty comparable, I think.
Some common/well known ones are as follows:
-Chem trails(Planes release chemical toxins into the air that do something?)
-The moon landing was faked
-9/11 was an inside job(Some people believe the well known terrorist attack was carried out by our own government.)
-There's a secret cure for cancer
-Lizard people run the government(people who believe this think that rich and powerful people are actually lizards and usually that all rich and powerful people are related in some way)
There are a lot more. I know people in my everyday life who believe the ones I mentioned, it's very widespread, and people who hold one conspiracy theory often times don't just have one.
As for why people in America are so likely to believe in these kinds of things, I think a lot of Americans have rightly placed distrust in our government and its institutions, so in a way having secret knowledge as to how the world operates might grant some form of comfort; just like superstitions used to.