Many online tools require registration. For users who just want to quickly jot down ideas, this is still a barrier. Fortunately, some tools don't require sign-up.
A notable newcomer is SmallMindMap, which has been rapidly gaining popularity. It requires zero registration and has no feature restrictions.
SmallMindMap delivers extreme simplicity: visit the site → auto-create a new map → start editing immediately. No sign-up popups, no trial timers.