Just a few minutes ago I responded to a Tweet from Meredith Stewart who was looking for some first day of school ideas to use with her 8th grade US History students. My suggestion was to try using Wallwisher to create a KWL (know, want to know, learned) chart that students can write on. Meredith already had that (or something like it) planned for day two, but that's not going to stop me from making a blog post out of this.
I've previously written about using Wallwisher with my special education students to create a collage of videos and pictures that they discovered and we discussed in class. Wallwisher could also be used to create a KWL chart that students can contribute to anonymously (or not anonymously if you want them to sign-in). Create a wall, make it public, and ask students to share what they know and what they want to know about a topic. If you allow anonymous posting you might get contributions from shy students who might not otherwise speak-up in class. Of course, if you allow anonymous commenting you should have a conversation with your students about what an appropriate comment looks like. (You could also turn on moderation and approve all comment before they appear).
Another option for creating an online KWL chart is to create and publish a Google Docs document. Create the document share it directly with students or its editing permissions to "anyone with the link" and invite students to write on the document. To keep the document organized you should insert a table that your students will fill-in.