This is a great free plugin that converts your wordpress blog into forum. I have actually tried it with my other site, it works nicely as a forum but unfortunately it is pretty hard to get a forum started and get the momentum going. Nevertheless, if you would like your wordpress login for your forum, this is the perfect free wordpress plugin for you.
There is also few theme/template that comes with Simplepress.
What I like best are the admin controls that allows usergroup for admins with different permission, custom smileys, posting editor, private messaging.

Best of all are the wordpress SEO that is inherited to this free Wordpress Forum plugin.
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We have also found another great plugin to convert your wordpress into a Classifieds portal, unfortunately they are not free. The single license cost $59 while the developer license (for multiple domain ussage cost $159. If you are interested you can check out the Wordpress Classifieds plugin.
Posted under Webmaster Freebies