TreeSheets (Hierarchical Spreadsheet)

TreeSheets is a free, open-source "hierarchical spreadsheet" created by developer Daniel Nauck (strlen.com). It blends the structure of a spreadsheet (grid of cells) with the outline/wiki-style freedom of nested, multi-line text, images, and other data — essentially a cross between a spreadsheet, a mind map, an outliner, and a note-taking app. Unlike traditional spreadsheets (Excel, Google Sheets) that enforce rigid rows/columns of numbers, TreeSheets lets users place any content anywhere in a tree-like hierarchy, making it useful for brainstorming, project planning, knowledge management, and personal databases. It has a niche but loyal following among power users who find conventional tools too restrictive, similar in spirit to tools like TiddlyWiki, Notion, or Roam Research, though it predates many of them (initial release 2008). The project is notable for being a one-person labor of love, built in C++ with a custom GUI (no web framework), emphasizing speed and portability (Windows, macOS, Linux).