According to CrunchBase "Edistorm is a social brainstorming application empowering friends and coworkers to rapidly brainstorm and make better decisions."
From the Edistorm website:
"Edistorm allows you to organize your ideas in a real time interactive wall. Each sticky note can have its own color and the users decide what the arrangement means to them.This freeform method of collaboration will feel familiar to anyone that has thrown stickies on their walls."
Edistorm is always free for anyone adding ideas to a brainstorming. Invite anyone.
Creating public or solo brainstorms is, while creating private invite only storms requires a subscription. All accounts have a no credit card required 30 day free trial. You can even follow them on Twitter to keep updated on how the product develops.