SocialEngine.net/ is social network software that helps you build your own customized community websites. Start simple, or go feature-rich with plugins.
Socialtext accelerates business performance by making it easier for employees to find the colleagues and information they need to solve challenges new and old. By simplifying people's ability to share expertise, ideas and corporate data, Socialtext removes knowledge silos that have traditionally hampered companies' ability to respond to change and serve customers efficiently. With Socialtext, people collaborate openly around key business processes in a secure, internal environment, and work together to drive new business opportunities.
Start exploring online community here. I am always collecting useful online community/social media/social networking sites, articles, resources, and experts, and I hope others find them useful. I will be updating this list regularly.