OpenGoo (http://www.opengoo.org) is an open source web-based collaboration 'portal', offering users access to Notes, Emails, Contacts, Calendar, Documents, Tasks and Events folders. More information can be found on the OpenGoo web site but for small businesses or even home users or students it is potentially a very nifty tool indeed.
'Workspaces' can be created, which can be private or shared, and users can be assigned permissions to other users' individual folders. It is very similar in functionality to a scaled-down version of Microsoft SharePoint.

The email component provides POP or IMAP access to your existing email infrastructure (this could be a GoogleMail account, or running locally in SquirrelMail or something similar), but can send email using SendMail installed locally.

Installation is quick and easy. The software requires PHP5, so if installing from scratch I would recommend CentOS 5.3 (http://www.centos.org): simply install a base configuration of the server software including the Web and MySQL roles.

The OpenGoo software itself is downloaded as a ZIP file which you can extract and put in the /var/www/html/ directory. Once extracted, make the subfolders writable with a 'chmod' command.

Create a MySQL database and database user for OpenGoo to use.

The installation is done via the web browser, simply browse to http://(opengoo_server)/public/install

Verify that all of the environment checks pass. Then enter the details of the MySQL database:

You're all set.

You will then be prompted to create an administrator account:

Once created, you will then be logged in to the main 'Workplace' as the administrator:

Users, Groups and Workplaces can be created by selecting the 'Administration' link in the top right hand corner.

Users can then create their own contact lists and edit their Calendar, Notes and Tasks folders in much the same way as any other PIM application:

Email accounts can be created quickly and easily by entering the details of the POP or IMAP server as well as the username and password of the email account:

Documents can be created directly in HTML with all the usual formatting tools available:

In short, a very nice tool and definitely worth a look for the Small Office, Home Office environment.