Google Groups - Google Groups allows you to create and participate in online forums and email-based groups with a rich experience for community conversations.
Mighty Networks - Mighty Networks enables entrepreneurs, organizations, and companies to create and grow a community-powered brand.
DownToMeet - DownToMeet is a platform to organize, find, and attend group events.
Meta Search - Search your Desktop, Google Drive, Dropbox, Gmail, Evernote.
Community Box - Easily create beautiful directories
Nabble - Nabble is a forum software that are embeddable and fully customizable with scripting language.