http://www.answergarden.ch/
Basics: Students repsond to a question of some sort with an answer that is less than 20 characters. All of their answers are recorded in the answer garden. The whole class can see what everyone else thinks about the topic. You can turn the brainstorming session into a wordle.
Example:
What is your favorite app to use in the classroom?... at AnswerGarden.ch.
Test Kitchen: Students love this app. I was having them brainstorm for an essay and they were putting in their answers to the prompt. They could not get enough of it. They loved that they got to see each other's answers. I turned their answers into a wordle and they were so excited to find their word in the wordle. This is a great way to brainstorm or share ideas and it gets students engaged in what they are doing.