Despite being one of the older and more traditional community platforms, PHP-Nuke continues to grow marketshare rapidly. The platform formerly known as Post Nuke is now called Zikula and is a fork of PHP-Nuke 5.0. Microsoft's Sharepoint is the first commercial product to make the list and is also one of the most mature and popular.
Rounding out the top four, Zikula is one of the older, more established offerings. Though Sharepoint can be used to develop collaborative environments that have few community features, the most recent emphasis and the majority of uses I encounter are for community-style deployments.
The rise of social networking and fundamentally user-powered Web 2.0 applications as a common daily activity has further helped move online communities into a mainstream business topic this year.
Social media -- and blogs in particular -- are also helping raise awareness of the power and reach of community-based communication and collaboration.
Those that can reach out, engage, and elicit useful participation over the network will increasingly have the advantage when it comes to achieving directed, collaborative problem solving, often known as Note that this also informs the Enterprise 2.0 story since community is a particularly important issue when we talk about the success of social software in the workplace.