The following free tools offer good options for creating mind maps online.
MindMup is a free mind mapping tool that can be used online, with Google Drive, and on your desktop. MindMup works like most mind mapping tools in that you can create a central idea and add child and sibling nodes all over a blank canvas. MindMup nodes can contain text and links. When you’re ready to save your MindMup mind map you can save it to Google Drive, save it to your desktop, or publish it online. If you publish it online, you can grab an embed code for it to post it in a blog post or webpage.
Lucidchart offers a simple drag and drop interface for creating flow charts, organizational charts, mind maps, and other types of diagrams. To create with Lucidchart just select elements from the menus and drag them to the canvas. You can re-size any element and type text within elements on your chart. Arrows and connecting lines can be re-sized, repositioned, and labeled to bring clarity to your diagrams. Google Chrome users can use Lucidchart offline through the Lucidchart Chrome app. Lucid Chart offers the option to include interactive tables and notations to your charts. You can right-click on any shape in your mind map to write and add a note about it.