Not a spreadsheet, no, (it is more of a freeform worksheet) but SMath Studio is a breeze to work on with units. It has complete support for units of measuremen

Not a spreadsheet, no, (it is more of a freeform worksheet) but SMath Studio is a breeze to work on with units. It has complete support for units of measurement and comes with most built-in. It can also decompose derived units into simpler forms if it finds a match. It is free but not open-source.