Myth: Mormons aren't Christian.
Fact: We definitely consider ourselves Christian, and most other faiths do as well. Our faith revolves entirely around the belief that Christ visited people in other lands after he came back to life, and that he still speaks today through prophets.
Myth: Mormons have the Book of Mormon instead of the Bible.
Fact: We actually have the Book of Mormon in addition to the Bible. We believe that Christ came to America shortly after coming back to life and taught his gospel to people there. They wrote his gospel just like his Apostles in and around Jerusalem and passed it down from generation to generation.
Myth: Mormons worship Joseph Smith.
Fact: We worship Jesus Christ. We think of Joseph Smith like Moses or Noah. We believe he was called by Jesus Christ to be a Prophet and to translate the Book of Mormon so we would have another Testament to Christ's divinity.
Myth: Mormons have multiple wives.
Fact: We don't. We did in the early days of our church, most likely because the women drastically outnumbered the men. But not any time recently, and in fact our very own Book of Mormon prohibits it in Jacob 2:24-30 except in special circumstances.
Edit: Fundamentalist Mormons still practice polygamy, but the rest of us do not.
Myth: Mormons are paid to knock on doors and spread their church.
Fact: I wish! That would be awesome! No, actually, it's quite the opposite. We pay our own money to go on our missions. It's about $10,000 to go on a 2 year mission. Some of that is definitely subsidized depending on where you go, but no, we are not payed to go on missions.
Myth: Mormons do creepy cult things in their temples.
Fact: We certainly don't. Like many religions we have rituals that we practice. These have sacred significance to us and are meant to be understood in terms of symbolism. We do not however do anything "creepy." We don't do any kind of blood sacrifice. We don't force anyone to do anything. We simply make promises to God through ancient rituals.
There are certainly more Myths about us, and if you have any potential myths, or questions, feel free to reply, and I'll do my best to answer! (I'll probably also add it to this post!)