Treesheets is billed as the "ultimate replacement for spreadsheets, mind mappers, outliners, PIMs, text editors and small databases." It is freeware and works on Windows, Unix, and the Mac.
"It's like a spreadsheet, immediately familiar, but much more suitable for complex data because it's hierarchical.
It's like a mind mapper, but more organized and compact.
It's like an outliner, but in more than one dimension.
It's like a text editor, but with structure."
It has a certain "geek-value," but I must say that the very unusual "box-within a box" metaphor does not click with me. It most like a spreadsheet, since it uses a cell-based structure. But each cell can contain large amounts of data. And you can zoom into these cells to reveal still other cells with more rows of information.
It's like a babuschka doll, except the dolls contained within the dolls can be vastly different from each other. This may be said to violate what some have called the "nested doll principle" or the structure of "similar objects-within-similar objects" found in some interesting natural and man-made phenomena. I, for one, find it hard to remember what complex data may be hidden deeper in the hierarchy. Treesheet's structure seems to be hiding deeper levels rather than revealing them. It is accordingly not suitable for long-term dat-storage.
But these reservations are probably just due to my shortcomings. I like my hierarchies relatively flat—at least during the discovery phase.