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Subsidiary concepts are arranged as sectors of concentric rings.
The earliest instance of this type (2003) appears to be UNO Universal Organizer, which I came across at the website of Metropolitan State University, St.Paul Minneapolis. According to its author, its origin was as a way of organizing a paper. This is a template for purely handwritten use.
When using software instead of paper, you have the greater convenience of being able to pick a sector and see that automatically transformed into the center of a new annular map, which emphasizes the focus of your attention at that moment.
One thing you may find inhibiting is having text at different angles with UNO charts or Goalscape.
These are multi-platform applications, desktop; on-line; and mobile (Crystal-mappings' mobile app is promised soon at the time of writing - Feb.2011).