ThinkFree Calc
http://online.thinkfree.com/

Google Spreadsheets
http://spreadsheets.google.com/

ajaxXLS
http://www.ajaxxls.com/

EditGrid
http://www.editgrid.com/

gOFFICE Spreadsheets
http://www.goffice.com/

iRows
http://www.irows.com/

Num Sum
http://www.numsum.com/

Sheester
http://www.sheetster.com/

wikiCalc
http://www.softwaregarden.com/wkcalpha/

Xcellery
http://www.xcellery.com/

Zoho Sheet
http://www.zohosheet.com/