A spread sheet wiki is a spreadsheet that is on a server, accessible through a web browser and editable like a Wiki is.

I don't know of any open source Wiki Spreadsheets, but here is a proprietary one:

http://web.peoriadesignweb.com/spreadsheet/

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TWiki (TwikiClone) has a Spreadsheet plugin - you edit the spreadsheet contents as plain text within the page.  You can also attach spreadsheet files, which some peole may prefer. --RichardDonkin

''Apparently'' you can also attach spreadsheets to JotSpot wiki pages.