Simple mind map is the best free mind mapping tool with no signup required. Works on all devices allowing you to create online mind maps across your desktop, phone (iPhone and Android) or tablet (iPad). I created this because all the other tools were too clunky, I just wanted to make a mind map online, quickly, and without needing to signup.
If you've used the tool and have any feedback, I'd love to hear it at contact(at)joshuaj.co.uk. Although I can't dedicate loads of time to the tool, I always want to improve it to make it the best, completely free mindmap tool by adding new features and squashing bugs.