SocialEngine.net/ is social network software that helps you build your own customized community websites. Start simple, or go feature-rich with plugins.
Disqus is all about changing the way people think about discussion on the web. We're big believers in the conversations and communities that form on blogs and other sites.
Connect. Share. Collaborate.
Elgg is free and open source software
It provides the components you need to build a fully-featured social environment, including:
A browser extension that lets you comment right next to any text or image that you find on the web. Currently only support Firefox 2 & 3 or Internet Explorer 7.
Twitter users of TweetDeck, check this out. Facebook Connect and Seesmic Desktop is a dream application for the frequent status updater, group lover, and social media meme tracker. Has multiple columns that can be rearranged via drag and drop, customizable groups, Twitpic functions, etc.
I started using Seesmic Desktop a few days ago and have found it to be much better than TweetDeck. One of the key features for me is that it allows you to manage multiple Twitter accounts.
Being number one boils down to this: people love to use Jive Social Business Software, which creates a major competitive advantage for customers who rely on SBS. Our next-generation communication and collaboration software is the only truly enterprise-scale social business solution on the market. Our foundation integrates the most powerful enterprise 2.0 features of collaboration software, community software, social networking software and social media monitoring tools to extend social capabilities across every level of your enterprise.
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.