While the Internet is populated with free forums, there is also the possibility to create a private one where members need to pay a single or recurring fee to join. These private forums could be one of the element build into your website or blog.

SEO Blackhat charges $100 monthly from its members, and they have thousands of them. Obviously in order to charge such a price for a forum membership you need to provide real value for the members (e.g., secret techniques, tools, and so on).

Performancing also launched a private forum, focused on the networking aspect. It is called The Hive, and the monthly cost is $10.
These are just two examples. There are many possibilities to create a private and profitable forum; you just need to find an appealing angle that will make it worth for the members.

However, there is a need to attract your reader to your blog before introducing such private forum.
Below show a list of popular forum software that you could make use of…

1. vBulletin

2. Simple Machines Forum

3. phpBB

4. Vanilla

Cheers!
Patrick