Wridea - online idea management and brainstorming tool

Wridea is a web service first to avoid ideas being forgotten, then to organize and improve those ideas by giving you the opportunity to share your ideas with friends and necessary tools to improve your ideas by yourself and individually. Once you've signed up and created a free account, you can add new ideas, categorize them, and share them with your friends using the very simple web interface.

It also sports an interesting developer API for third-party add-ons, and a unique "Idea Rain" visualization that drops your ideas into the screen