Google Groups is Google's Web-based answer to e-mail discussion groups. It integrates forums, file sharing, group e-mailing, and collaborative document creation to take the discussion away from your e-mail client and put it in your browser.
There are thousands of groups on Google, and if you can't find what you're looking for you can start your own. If you've got a Google account, Google Groups will simply reuse some of that information to create a profile. Groups users can also custom tailor the information they share with others for each group they're in.
There have already been several interesting uses of Google Groups, including company tech support pages and product knowledge bases. Like other Google projects, users have taken the tools provided and bended them to match.
Winner: Google Groups (Groups.Google.com)