Google Spreadsheets, the online spreadsheet program that is part of Google Docs, has just added a new feature by which you can make a spreadsheet open for editing to anyone and everyone. Before, you could invite collaborators but you had to provide them with a link that included a secret code so only invited people could participate. Now you can have it open for anyone to click a link from a web page or blog and edit in real-time with many others. See a full explanation at http://googlesystem.blogspot.com/2008/05/google-spreadsheets-become-wikis.html
Cool.