Casual

Casual is an online project management tool that differs from the rest. It helps teams plan and execute projects as simple workflows online. source

If you are looking for a very good yet simple tool to plan and organize your thoughts on a particular project, casual is your choice. You do not have to be a trained project manager before you can use this tool and a lot of users have been testifying to its awesomeness. You can watch the video below to gain more insight into this app.