People from (usually) Christian faiths who go knocking door to door spreading their view on Christianity (or likewise). They most common variety are the LDS (mormons) and Jehova's Witnesses (Jay Dubs).
They can easily be identified, the LDS Missionaries always wear either suits or white shirts, ties, and slacks and always come in pairs of two, and in rare occasions three. The JW missionaries usually come in a car with at least two (sometimes a full car) and only one will knock on each door (as compared to LDS missionaries who both go to the same door at the same time). Also, LDS missionaries are usually 19-21 year old men, occasionally women of the same age, and in even rarer occasions elderly couples. The Jehova Witnesses come in nearly the full spectrum of human age.
Contrary to popular belief, not all mormon missionaries ride bikes. Some of them get cars, and some only walk.
I put a "NO SOLICITING" sign outside my door to keep those pesky missionaries off my property.