Once I decided to start a discussion forum on my wordpress blog to make my site a complete resource for blogging and web development tips. There are many forum scripts available on the new – including the free PHPB3 and the paid vbulletin scripts.
I thought there must be some wordpress forum plugin for this and to my surprise, there were many. 
That makes wordpress not just a blogging script – thats a complete CMS..
Here are the 2 well knows wordpress forum plugins
Hope you will find them useful. If you know more such plugins, let me know. I will add them to this list. Happy Blogging ! 
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#1 by Tushar on November 29th, 2008
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Nice collection, and you’re right, WordPress is a complete CMS. Honestly, there are so many plugins and mods and themes for it that you make it virtually anything, from an auction site to a news site to a professional portfolio.
Read Tushars last blog post..td.NOV – The Ultimate Blog Contest