Google Sheets - Synchronizing, online-based word processor, part of Google Drive.
SSuite Axcel Professional Spreadsheet - This powerful and professional spreadsheet has all the tools you need to calculate, analyse, summarize, and present your data in numerical reports or colourful graphics.
EtherCalc - EtherCalc is a web spreadsheet.
Gnumeric - Gnumeric
Keikai - Effortlessly Build Spreadsheet-driven Web Apps
LibreOffice - Calc - LibreOffice Calc is the spreadsheet program you've always needed. A fork of OpenOffice.