I've never fully understood why EllisLabs opted for building a completely separate module for forums when the structure of ExpressionEngine can be used to create fully bespoke forums.
You have a channel called forums, pre-defined categories for sections of the forum - so mimicking EE's own forums, you have a parent category of "ExpressionEngine 2" and 2 child categories "Community Support" and "Development & Programming". Then using frontend entry forms, allow users to post (either as a visitor or ensure they've logged in first). When a user creates a "new topic", they have to select from your custom form submission a relevant category and it creates an entry. Then anyone wanting to reply, it's a comment.
As the official EE forums, you can display the author, the total views, the total replies and the last reply (comment). The search can be built to only target that channel.
You can set it to automatically email the owner of the topic and obviously when someone replies (comments), they can also subscribe to alerts. You can even allow people to subscribe to a thread (entry).
You can use richer third-party wysiwyg editors, instead of the basic EE one, you can add better image upload, with more controls, you can utilise smiley implementation and much, much more.
All the above inclusive of the original EE license without additional cost. I'm not sure how much extra effort would be involved in doing it this way versus native forum module, but considering the difficulty in customising the forum's appearance and the control you'd get from native entries, the EE template route would win me over every time.
And using this add-on, you could even allow for threaded
comments - sorry topic "replies".