Collaborize Classroom allows students to collaborize and work together on brainstorming, discussions etc.
Bubbl.us is a very simple interactive mind mapping tool and these videos outline the different tools which make it such a good tool for students to use.
If you are working with groups of students and you want them to brainstorm on ideas or collaborate with students then you might want to consider this tool. The interface is great and so easy to use.
Today's Meet allows you to have live feeds in lectures, conferences and lessons and is so easy to use. These videos show how students can listen and add comments from different locations.
In Tricider you can write a question, generate a weblink and then share the weblink with your students and they can click on the link and add their answers. The format is really nice and it means you can also add a question, write a series of suggested answers and get the students to comment on them.