WikiCalc

Spreadsheet in a Wiki, written by Dan Bricklin

• acquired by SocialText, renamed Social Calc

competing with Google Spreadsheet

Feb'2013 update http://www.bricklin.com/javascriptrollson.htm: I've also heard of other uses of the SocialCalc code, such as in the Sheetnode module for Drupal, and Audrey Tang's multi-user EtherCalc.net. (Audrey was my main technical partner at SocialText in the latter parts of the project, and she documents the internals at that link.)