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there's lots of Open Source CMS out there, mostly based on the LAMP (Linux, Apache, MySQL, PHP) architecture, but a few for Windows and even for Macintosh. Among the most commonly used, with numerous of installations (and therefore a quite active userbase) are:
I would recommend to look at one of these three, but as has been said before, there's zillions of others.
There are many CMS out there but its important what you expect from a CMS?.There are CMS's for built a website and some for blogging .As a long time CMS user i care;
I would tell you beware of any x nuke CMS ,i can suggest you have a look at
Typo3 , (if you have the requirements )