Although not technically a CMS, Invision Power-Board (IPB) has become a very popular peice of software. It is a message board system that boasts extensibility and grace under pressure. IPB has useful features like the ability to define a homepage layout, as well as powerful community building features to help bring excitement in the vast world of forum systems.
You can extend Invision with plugins from third party developers. The IbProAracde plugin is just such an extension. It allows you to have a full featured arcade right in your forum. Although installation and upgrades of the extension are not as simple as true CMS modules, instructions are provided that make the process easy to perform.
Sample Invision Plug-in installation
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To download Invision, Visit the Invision Homepage