I'm speculating here, but it seems to me that the most common misconception about atheists involves the meaning of atheism itself and centers around the belief that an atheist is someone who is 100% certain that one particular god does not exist. I have heard this from Christians countless times. For many Christians, an atheist is someone who is completely certain that the Christian god does not exist. Obviously, this can be criticized on the grounds that atheists don't believe in any gods and trying to tie it to one particular god is absurd; however, that really isn't the point. The point is the issue of absolute certainty.
An atheist is someone who does not accept the theistic belief claim (i.e., some sort of god or gods exist). In other words, atheists do not believe in any gods. Atheism does not entail any sort of certainty or absolute knowledge of gods. Some atheists will say that they are certain no gods exist; they are referred to as gnostic atheists. Other atheists will say they are not absolutely certain that no gods exist, and they are referred to as agnostic atheists. The same is true of theists, by the way, and you can find a helpful diagram illustrating this here. Both gnostic atheists and agnostic atheists are still atheists in that they do not believe in gods.
Some misconceptions persist primarily out of ignorance; others persist because they serve a purpose that those holding them find advantageous. I suspect that the misconception about atheists being absolutely certain that gods do not exist may be a case of both. What sort of advantage might such a misconception confer upon the modern Christian? Most of the time I see Christians citing this misconception, it is in service to the argument that the atheist is irrational for holding a belief without evidence (i.e., "I know your god doesn't exist") in the same way the Christian is irrational for holding a belief without evidence (i.e., "I know my god exists"). When the atheist criticizes faith, the Christian claims that atheism also requires faith and then lobs this particular misconception onto the field.
I am an atheist, which means that I do not believe in gods. I do not believe in gods because the theist has failed to meet his or her burden of proof. I regard the evidence provided to support the existence of gods as being insufficient, and so I do not accept the belief claim. I recognize that this could change. Some lucky theist could come up with the sort of evidence that would be sufficient to support the god claim, and this would lead me to re-evaluate my position. I am not at all certain that no sort of gods could possibly exist; I simply am unpersuaded by the evidence that they do. Thus, I am an agnostic atheist who would be happy to be proven wrong on the question of gods.
