Having looked at over a 100, I conclude that TUTOS is simply the most advanced GPL groupware/intranet/project manager available now.
Not very good looking or flashy. However, you can use & design alternate themes to jazz it up.
The menu system could be more intuitive. However, it's possible to activate only a few modules for certain users or groups of users, making it simpler.
-Absolutely superb permission & group system which gives you a precise control over who see & can edit what.
-Plenty of modules (calendar, bug tracking, file management, notes, tasks & projects, watchlist (notify of changes), resource, timetrack & invoice, fax, sms, addressbook, mail & LDAP, and others...
-Currently #2 on SF.net
-I disagree with previous post about "low flexibility": Tags can be easily modified in the language file.
-URL bookmark manager
-Global search box
You should definitely install and play around with TUTOS. Another great choice is phpgroupware.org