Yahoo Groups provides an easy and simple way to create online groups using nothing more than your Yahoo account. It's completely free, and offers users a place for group photo albums, calendaring, forums, and link sharing. It's great for making dead simple work groups online, and if you're feeling like some entertainment, you can join one of the publicly created groups that are available to everyone.
The groups on Yahoo run the gamut from simple fan gatherings for celebrities, all the way to tech support for commercial products. What gets the most use, however, are the integrated forums, which lay host to huge multithreaded discussions from group members. Often the content from these posts end up on other Yahoo sites, such as Yahoo Answers or Flickr.
Winner: Yahoo Groups (Groups.Yahoo.com)