MS Exchange alternatives, work-alikes, ...

• http://scalix.com/
• http://www.zimbra.com/

Also, see the "Help me avoid Exchange" thread at http://mail.gnhlug.org/mailman/private/gnhlug-discuss/2005-December/thread.html

Office Automation

• http://www.openoffice.org/ - MS-Office - like suite

Groupware

Typically web-based portals, Personal Information Managers, ...

• http://www.phpgroupware.org/
• http://www.horde.org/