A few former Mambo developers, including at least 3 development team leaders have left the team to start their own CMS - Mia.
http://miacms.org/
Having been on the Mambo team myself for a while and having worked with a few of the devs who are starting this project, I have to say I am pretty excited to see where it will go.
All of the developers are very skilled and have some great ideas for the future of their CMS.
To help them along we will be adding Mia to our list of supported CMSes soon and I encourage you to check it out when you have a chance.
Being based on the Mambo 4.6.x code, which is almost identical to Joomla 1.0, I think most of our Joomla 1.0 extensions will work straight off with no modifications necessary, as will most other Joomla 1.0 extensions.
Nothing like some healthy competition to drive the quality up of CMSes in general.
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