Dotstorming is a free service that can be used for meetings or in the classroom. Each session starts with a board, and items are added to the board. On the right side is a chat box that can be used for a back channel.
Each item in the board can be voted upon by the participants, allowing for a sorting of the best and worst ideas.
Sign-up was quick and easy. Each board gets a special URL that you can then use to share with students or participants in the meeting.