Usenet is a discussion service that can be read through a news reader (kinda like group e-mail); it's as old as the Internet and very well used. There are thousands of groups for all kinds of topics, and you can make your own group if there isn't one on your subject already. Plus, the vast majority of groups are unmoderated, so while you may have to put up with spam and trolls, you can say whatever you want.
I've enjoyed it for 3 years now. It's a lot of fun if you find the right topic to discuss :)