The earth is flat -- I don't personally know anyone who believes this, but I saw an interview once.
We never landed on the moon -- this is the Bad Astronomer's pet peeve, but I've run into one or two myself.
The CIA (or some other government agency) shot Kennedy -- any conspiracy theory that involves the CIA is immediately suspect. Is it possible? Sure, I suppose so. But given the number of possible conspiracy theories that involve the CIA and keeping it secret without letting any leaks out... the likelihood that most of them are true is virtually zero. You know how hard it was for Obama to keep his cabinet picks secret? What do you think the average secretary would do if they ran across this data?
There is some secret something behind the 9/11 attacks -- some secret something to get Saudi citizens out of the country after the fact? Maybe. But the US government planning the attacks? The more insane these claims get, the less likely they are true. If it wasn't for the evidence we actually have, I would have not believed planes could fly into buildings. That's not crazy enough for you?
UFOs are aliens from outer space -- see the comment on the CIA about the secretary. Give me a break. Strange that astronomers don't seem to see these aliens from outer space in any of their telescopes.
Vaccines cause autism -- got into a screaming match with family this weekend about this one. After Wakefield was proven to be a fraud, I'm still defending myself against all the evidence that says there is no link.
Astrology is real -- I'm nothing like a Capricorn. Strange that astrological signs don't follow the precession of the equinoxes, isn't it?
Creationism -- of any kind really; the mountains of evidence that supports evolution (and the age of the universe, I might add) is really unquestionably better than what is in the Bible, and far more credible. Can you imagine how credible it would be if someone took Greek creation myths seriously?
Climate change denial -- the climate is really changing. The theory isn't 10 or 20 or even 30 years old. It's 114 years old. There was enough data to propose it in 1895. We've had more than enough time to gather evidence and we have mountains of it. Every week I see at least two new research articles about more evidence that says Earth is warming and it's getting worse faster than we thought.
Obama was not born in the US -- how much evidence do you need really? Even if we had videotape of the actual event, how many people would be screaming about it being faked?
Obama is going to make schools (and federal service organizations) militaristic and brainwash our kids -- jesus, I didn't even think Bush was going to do this! Where did it come from?
Obama is a socialist -- more nonsense. I'm left of Obama and I'm not a socialist yet. Deal with it.
Labour unions are the cause of the economic collapse -- more right wing nonsense from people who don't understand history or economics.
America is a Christian nation -- again, someone with no understanding of history, and who clearly hasn't actually read the Constitution.
I'm sure there are more, but these are the big ones.