Gridpark is a free, open-source desktop spreadsheet that reads and writes real .xlsx files, reasonably compatible with Excel. Powered by HyperFormula's 400+ formula functions. A full styling toolbar covers fonts, colors, borders, number formats, and cell merge. The twist: you can draw directly on your spreadsheet. Pen, highlighter, spray can, eraser, annotate your data like it's a wall. Dark/light themes, Japanese + English UI, sort, filter, freeze rows, undo/redo, autosave.