Vanilla is an open source and free forum platform if you host it on your own server. They offer a cloud hosting as a paid subscription.
The platform looks modern, but does not have included many common options that other free or paid platforms have.
The have an active add-on marketplace where you can find different plugins to extend the functionality.
Vanilla is not so popular as other platforms, but comes with a new look and feel that you could like. You can use it for free and you can subscribe on their cloud plans if you like it.