TreeSheets is a freeform data organiser, which the developer describes as "the ultimate replacement for spreadsheets, mind mappers, outliners, PIMs, text editors and small databases".
The program opens with a grid, which can be set to whatever size you like. At its simplest you could just start to type, filling each cell with text notes, or inserting images.
What's more interesting is TreeSheet's support for inserting grids within grids. This allows you to build hierarchies, like a todo list, where one item contains a further set of items you'll need to complete.
With a little work, these can be surprisingly flexible. For example, you might have organised a project plan by milestone, then some key element, then team members, and finally the subtasks they're carrying out. But in a click or two you can reorganise this tree to, say, display it from the perspective of an individual team member, so now they're at the top of the hierarchy, and you can instantly see which tasks they're doing.
More generally, there are lots of navigation, formatting and presentation options to help ensure the program works for you (check out the screenshot for some examples of what you can do).
Your work may be saved at any point as XML, CSV, HTML, indented text, even an image.
Or, if you have the data already, TreeSheets will import XML, CSV, tab-delimited text and more.
TreeSheets is a well-designed and versatile way to organise your data. There's a lot to learn - if you just need basic todo list management then you'll be better off elsewhere - but more demanding users will find plenty to like here.