A modern Excel workbook, which is really a zip full of XML. Unlike a CSV it knows what a number is, holds multiple sheets, and keeps formulas and formatting. It's what people mean when they say 'send me a spreadsheet'.

Every conversion below runs in this page, on your own machine. Nothing is uploaded, there is no queue, and there is no account to make. Pick a target format, or just drop a XLSX file above and choose afterwards.