Google Spreadsheets added an option in the sharing dialog that allows anyone to view or edit the spreadsheet just by knowing the URL. Until now, you had to send an invitation URL that contained a secret code and the people you invited had to login using a Google account. If you click on the Share tab and enable "Anyone can edit this document WITHOUT LOGGING IN", your spreadsheet becomes a wiki that can be edited by anyone.
To experiment the new feature, I set up a spreadsheet that compares the features of two desktop office suites (MS Office, OpenOffice) and two online office suites (Google Docs, Zoho). The spreadsheet has three sheets for: word processing, spreadsheets and presentations.
In other spreadsheet-related news, you can now embed forms in a web site by just copying some code, there's a new option to duplicate questions and users can add their own answer to a multiple-choice question.
Update: You can view a snapshot of the spreadsheet after the first day of editing. The comparison has been extended with information about voice recorders, clay tablets, smoke signals, telepathy, Jedi mind control and more.
Update 2 (May 26): You can no longer edit the spreadsheet.