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Yes. A new version of Xoops....
Seriously though, the big four of the open source are in general considered to be (in no particular order) Drupal, e107, Mambo and Xoops. What each of these means to you will greatly depend on what angle you're coming at.
I actually installed all 4 of the above on a test system last week to have a look around. I'd consider myself to be an experienced to advanced user at best, and I really do think XOOPS still is one of the best interfaces. I do have to give credit to Mambo, as it REALLY does have a very polished feel to it from a user perspective, and in comparision I found the XOOPS interface quite drab and tired in the eye candy department. However, looks alone do not make a good CMS. I found e107 quite good too, but found Drupal a lot more complicated than it should be to install a CMS (it has no automated install process).
A developer might have a totally different view as they are usually more concerned about the structure rather than the user interface.