On Dotstorming you can have people submit ideas in the forms of text, image, and video links. All submissions appear in a grid where viewers can then vote and comment on the submissions. Recently, Dotstorming added a couple of new features that teachers will find helpful.
Create a pinterest-like board, participants can add their own ideas, youtube videos, images, etc. Vote on the ones that you like the best. It's like combining padlet, pinterest, and the now defunct Google moderator.
I've used many mind mapping tools and find them lacking in functionality (xmind) , expensive (inspiration) or very hard to use (Free Mind). Most of these tools will not allow you to have 2 main topics and create a double bubble map (like a venn diagram). Edraw Mind Map has a very easy to use ribbon interface similar to Word 2010 and tons of built in symbols and mind mapping templates. It is free for educational use and their site has tons of video tutorials that cover all the nifty features. It's the first mind mapping program where I felt like I wasn't fighting the interface.
Wikibrains is a social brainstorming tool that helps you think. It's fast, visual and fun. Put in a word or topic and see what others have brainstormed or start one of your own.
Solvr is a free to use website that lets us post a problem and then share that URL with our friends whom we want to ask the solution from. This can be helpful when we want to reach a solution as group. The approach is that of an online forum but dedicated to problem solving.