Collaborize is fabulous. It allows students to vote and comment. It will also generate graphs of students' votes. Super easy to use. You can embed pictures and videos as well. Here's a blogpost of how I first used it in my class.
Here is a list that, I believe I got from Tech and Learning :-) I have also begun using Google Sites for quite a bit of my collaboration. It's a great place for students to collaborate and create a project!
Top 10 Sites for Brainstorming/Mind Mapping
1. Slick Plan - What's not to like? Easy to use, create beautiful flow charts, then embed to a site.
2. Thinklinkr - Real-time collaborative site where users brain storm and create a mind map.
3. Webspiration - The popular desktop application is now the web... nuff said.
4. Diagrammr- Quite simply the easiest mind mapping site to use, no registration is necessary, then share the URL w/ others.
5. Imagination Cubed- Nice looking site that is ideal for the electronic whiteboards w/ the ability to draw your own mind maps.
6. Edistorm- Real-Time mind mapping w/ sticky notes.
7. Education Eye- A great way to brainstorm w/ this innovative search engine.
8. Mind42- Nice collaborative mind mapping site that is easy to use.
9. Mindmeister- Beautiful looking mind mapping site w/ the ability to embed into a site.
10. Mindomo- One of the most detailed mind mapping sites w/ lots of web 2.0 features, such as sharing, commenting, etc.