You may have heard about Mind Maps. Basically, it is to help you connect ideas on the one main concept. The main concept is at the center and you branch the related ideas from the center. It is ideal for brainstorming on any topic.
What about creating them online?
There are lots of alternatives on the internet, if you do a little research. You may want to choose one that fits your needs better.
In my case, the best solution was examtime.com. Simply put, it is free and it also allows me to set the project status to Private. Considering you use Mind Maps when you generally brainstorm, it is less likely you may want to make it private. Other tools I checked briefly offers similar features with paid packages.