Nexo.com is a new collaborative service which is being used by nonprofits to improve communications, participation and documentation. It can be used by nonprofit boards, by volunteer groups, by staff or any other group.

Nexo is a free and easy service for groups which combines rich website creation, group email communications, social networking and real-time collaboration in a secure environment. Nexo has widgets for sharing calendars, tasks, files, feeds, polls, blogs, pictures, bookmarks and more. Nexo is highly customizable, has fine-grained permissions to ensure appropriate privacy, and is dynamic and open.  

If you need to share information with volunteers, board members, and other levels in your organization I would suggest OFFICEZILLA.COM.  Which is a free groupware system that allows you to add users, give them permissions, allow collaboration (or not) and just do all sorts of things all free.  It also has RSS feeds for every area.  So if a new message is posted, a new file, a new calendar event and so on everyone can know about it.

<P>http://www.officezilla.com/

If you are looking for an open source tool that is helpful for Project Management, you might want to take a look at DotProject .  I've found it very useful for project management and communications. 

Steve Hanson Principal consultant Cruiskeen Consulting http://www.cruiskeenconsulting.com

Are you talking collaboration internally within one organization or collaboration that is boundary spanning (beyond org's walls or a distributed team?)

 Nancy White is working on an interesting report about technologies for communities of practice and she recently some concept maps of the thinking on flickr -- it is about collaboration tool selection and mentions some web20 tools.

The Complete Set Is here
http://www.flickr.com/photos/choconancy/sets/1638079/ 

It won't give you a specific answer in terms of the name of a tool, but does outline a decision-making process -- mapping needs/contexts to categories of tools.

For collaboration - there's basecamp.com -- Marnie Webb wrote about it on her blog http://www.ext337.org.   I didn't think the learning curve for the software itself was difficult, but it does as Marnie notes in her post turn traditional project on its ear -- so there might some change issues to adoption.

 There was also a thread about project management tools on the riders tech list not too long ago

http://npogroups.org/lists/arc/riders-tech/2005-12/msg00028.html

(You might have to be a member of the group to view the archive)