Formerly Wallwisher, Padlet is a great interactive online board that students can post to. Although there are many different interactive boards available, this one is my favorite because it's easy to use, it can be customized and it allows teachers to moderate posts before they appear. I also have never had any issues with the site itself.
Although I've used it several different ways, one of the most frequent ways I use it is to post a guiding/essential question for a unit, often as an introductory activity. The question, "What Was Life Like on the Great Plains in 1870?" is the question I post when I start my westward expansion unit that focuses on Native American conflict in the 2nd half of the 19th century. I require students to post a picture or video along with their response also. I save their boards for them to use to review at the end of the unit as well. This is just a very small sample of a student response board. They are much more full, it was just difficult to include a thorough picture of a board because most students posted with first and last names.