When it comes to great forum scripts, you have a lot of choices out there. One forum script that is receiving a lot of great reviews is MyBB. It is a free and open source alternative to other forum scripts such as vB, IPB, phpBBB, SMF and more.
Here is a little more about MyBB from the developers:
A lot of thought has gone into the MyBB interface to make it easy to use. MyBB uses a standard discussion board structure, so your visitors will feel familiar with the way MyBB works. Your visitors can use the MyBB user control panel to change the way they view discussions. They can also easily start and reply to discussions and markup their posts.
From square one, MyBB does a lot to give you an interface that is not just simply designed but is also familiar to use. You can change things extensively from within the administration panel. The ability to edit the layout, options and behavior of the script is all located within that area. The MyBB forums are also very extendable. It has a very advanced plugin system that allows you to tweak almost every aspect of how the forum works and feels to your users.
Already using another forum script? Their MyBB Merge System will allow you to easily convert your posts from another forum script over to MyBB. If you need more information, be sure to go check out mybb.com.