The downside of any free CMS is that you're limited to being stuck with what it gives you. Commercial ones get very expensive to modify, and you may well be better off getting a system designed specifically for your needs. Don't specify things you don't actually need though! References :
Why don't you make your own? stop stealing other people's programs because your to lazy to make your own.
References :
The most popular one at the moment seems to be Joomla (http://www.joomla.org/). Many hosting companies provide it as part of their web-hosting (we do in the UK for example). Other popular CMS are http://www.xoops.org/ , http://drupal.org/ and http://www.cushycms.com/ , and you'll find each have their fans and detractors
The downside of any free CMS is that you're limited to being stuck with what it gives you. Commercial ones get very expensive to modify, and you may well be better off getting a system designed specifically for your needs. Don't specify things you don't actually need though!
References :