Brainstorming is a very effective technique to generate ideas. To understand what exactly brainstorming is and how to use it in EFL and ESL classes, have a look at these previous posts.
Brainstorm on line
In this post, however, I want to share with you this website which both students and teachers can use to brainstorm online:
This tool is a free, easy and interesting way to collaboratively brainstorm and attempt to solve problems online. A detailed description of how it works is given by Nik Peachey.