TreeSheets is a tree-oriented, hierarchical spreadsheeting program, used to brainstorm or build structured data. It's written in C++ and its interface is powered by wxWidgets, which allows it to be multi-platform. It's available under the ZLIB license.

Shift + Enter

Adds a new cell inside/under the current cell.

Ctrl + Mouse Wheel

Zoom in/out.

Shift + Mouse Wheel

Increase/decrease text size inside currently selected cell(s).