Padlet which was once upon a time known as wallwisher is an excellent little app which allows you to create an online pin board. Boards can support text as well as image and video based pins.
Once created a pin board can then be shared with others to allow them to post their own comments or pins.
Pin boards can be set up to be totally public or can be set up such that contributions need to be approved before appearing publically to people visiting a board.
I am currently making use of Padlet to gather student feedback asking for their comments and suggestions. I have also previously made use of Padlet as a back channel for questions or comments on learning. This is even more useful where you are in a 1:1 mobile device environment so that students can post their questions and comments as they arise within the lesson. Where you also have a secondary display this is made even more useful as you can easily see the questions and comments as they arise therefore allowing you to adjust and modify the lesson based on the contributions made to a Padlet via your students.
Padlet can also be very useful to prompt discussion of open ended questions by posting the initiating question on a board and then asking the students to then post their comments on the board, plus to comment on each others postings.
Overall Padlet is quite a simple app in terms of its functionality however with a little bit of creativity in its use, it can turn out to be very useful indeed especially where you want to promote collaboration as well as discussion and debate.