I am participating on different forums and some of them are built with this open source software called: Discourse.

From usability point of view it’s the best one I am using. It’s not only the look and feel, it’s the sequence of information offered, the access to the already read threads, and many little details that makes you feel comfortable using it. You feel you save a lot of time when reading a blog using this software.

This behavior is on the desktop and mobile, which makes quick reviews of updates very easy. The keep you engaged on the web.

On the .org page you can implement it for your own forum with different pricing options.

Tribe.so

You can use https://tribe.so/ too for installation of an environment.