An easy way to quickly build a crowd of loyal visitors to your site is by launching a discussion forum.

Here are links to some forum software providers:

VBulletin is a powerful and fully customizable forums package and a good solution for all medium-to-large sites.

phpBB is a free PHP based forum software package suitable for smaller forums.

InstantForum.NET offers 'InstantForum.NET v 4.1' which  is a high performance ASP.NET & SQL Server discussion forum.  

Google Groups is a free solution presented by Google.

Find more information about Forum Software by going here.