Some time ago Google acquired the wiki site, JotSpot, and has been developing it ever since. Now it has been re-born within Google's new application, Google Sites, which makes "creating a team site as easy as editing a document".
"People can work together on a Site to add file attachments, information from other Google applications (like Google Docs, Google Calendar, YouTube and Picasa), and new free-form content. Creating a site together is as easy as editing a document, and you always control who has access, whether it's just yourself, your team, or your whole organization. You can even publish Sites to the world. The Google Sites web application is accessible from any internet connected computer."
Google Sites is available from within Google Apps. Each Google Apps account receives at least 10GB of storage in Google Sites. Google Apps Premier and Education editions get an additional 500MB for each user account.
PS I wonder who will be the first person to mention it as one of their Top 10 Tools? !!