There is also Dan Bricklin's socialcalc https://github.com/DanBricklin/socialcalc which has some relatively active forks including audreyt's ethercalc https://github.com/audreyt/ethercalc

You can see what the spreadsheets look like at http://www.ethercalc.org/

For just viewing and editing spreadsheets without social features, you can ignore the servers included, the javascript widget works just fine by itself.

Dan Bricklin co-authored VisiCalc, the first spreadsheet program for the masses.