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Check out
http://www.opensourcecms.com if you want to see what is available. We'll tell you about XOOPS right here
XOOPS vs. Mambo and PHPNuke, is in my opinion quality vs. eyecandy and quantity.
XOOPS is high quality coding, very stable, flexible and powerful. It's object orientation allows developers to expand the system easily, using the advanced kernel and core.
Mambo Open SOurce is relatively new, but has advanced real quick. It has lots of eyecandy, but under the hood XOOPS is far more advanced.
PHPNuke is the biggest of all the *Nukes, with so many members, in-house forks, compatibility issues, addons, versions, hacks, you name it, that it is unmanageable. Their focus is quantity.
I'm sure you'll find many many many more posts like this if you do a search on the forums (you're definately not the first to ask this), and many other community members will comment you on the pros and cons of XOOPS.
Herko