Wallwisher is a great tool for brainstorming and getting students or participants to reflect on a topic. Basically the page consists of a wall where users can add sticky notes or post comments, images, video links, etc. to address a question, problem, or discussion.

Registration is free, however, only students who are 13 years and older are allowed to create accounts. Truly, the only one who would need an account is the teacher because students can respond to the main topic by adding a post (no login required).

Resources:

How to Use Wallwisher

Wallwisher Samples

Sample: