Yahoo, MSN, AOL, etc are loaded with forums covering just about anything you can imagine. In addition, www.ezboard.com hosts thousands of forums and the option to create your own forums free of charge but your forum would be swamped with ads and pop ups. For a small fee, you can have the ads and pop ups removed.

You can also use IRC or a news server provided by your ISP to chat in literaly tens of thousands of discussion groups.