MotherJones did an interesting round-up of conspiracy theories about Obama earlier this month. Most of them are pretty normal: "Born in Kenya", "Secretly Muslim", "Is the Antichrist", "Is a Lizard."
But some are so orthogonal to reality that it may be impossible to understand their origins. I was incredulous at first, but MotherJones provided links demonstrating that each of these conspiracy theories is actually believed by some. Click through to these sites only after donning your combat rated facepalm helmet.