Completely free... the site is funded through ads. There are a few features that are enabled as you've become more active as a member. For example, after 100 posts, ads are removed. After 300 (I think it was) posts, you have the ability to create Resources (like an article). But the forums are free.
One exception... no advertising stuff for sale. There is a dedicated forum for that. It does cost a measly $5/mo to be able to post new threads to that forum. It's not very active, though, but available for those who want to post something for sale.
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