what is the best free cms (content management system) for multiple users?

depends on what your particular needs are. why don't you tell us what
exactly you look for in a cms and i'll give you some ideas.

I can suggest my 2 favourites:
PHPnuke: www.phpnuke.org
Mambo: www.mamboserver.com

Best regards
Claudio

I rather like Zope+Plone.  http://plone.org/
Plone is very user friendly and it scales well for large orginistions
if it needs to.

There is also another good one called typo3.  You can find it at
http://www.typo3.org

Thanks,

BrettWilcox