Integrates Vanilla Forums with WordPress: embedded blog comments, embedded forum, single sign on, and WordPress widgets.
Once you've worked out how this plugin works, and what its dependencies are, it works a treat. Contrary to some comments on here, you don't need to sign up for a paid hosted forum. You can, but you can also use this with the free open source GPL, self hosted Vanilla forum software that you can download from vanillaforums.org.
In addition to installing this plugin in wordpress, you will also need to install a plugin for vanilla forum called jsconnect. This plugin will not work if your vanilla forum does not have that installed. I also installed a plugin called 'jsconnect auto', which automatically logs a user into the forum if jsconnect detects they are logged in to wordpress. This makes for a smoother user experience if you have a 1-to-1 SSO setup. If you want your forum to support SSO from multiple sources, you may not want that.
Next, you might get an error in your wordpress config, under Vanilla forum -> Setup, which says
Couldn't find a Vanilla Forum at this url :/. If all you want is the SSO, ignore this and try setting up the Vanilla forum -> Single Sign-on page. This worked for me even though I still get the above error. I didn't bother with the embed forum feature or the comment integration. One of the three widgets doesn't seem to work. Maybe this is related to the Couldn't find error.
This is one of the best forums that I ever see and works great on wordpress.
My only complaint is that the forum is a little complicated.