Collaborating in an online environment can take shape in many different ways. It can mean the creation of a document with colleagues, it can involve a discussion, booking and scheduling dates, publishing to a website, charting progress and sharing information. There isn't an organization or company that doesn't need to collaborate on a regular basis. The problem is that most don't have the tools to do so. Open source applications open up the possibilities and make collaboration possible whether you're building a community or just need to have a long distance meeting.
Open Atrium is an intranet in a box that has group spaces to allow different teams to have their own conversations.
TikiWiki (Tiki) is a full featured Groupware/CMS (Content Management System) solution.
Jabber.org is the original IM service based on XMPP, the open standard for instant messaging, and one of the biggest nodes on the open Jabber network.
