I've encountered a number of Content Management Systems (CMS) in recent web based projects, both commercial and open source. Many of them can cope with minor tweaks, but even the open source variants cannot be easily modified to add significant features (I'm looking at you Alfresco :)). Or maybe this is true with just the Java open source variants (I haven't tried some of the heavily used PHP CMSssss like Joomla).
My thoughts on CMS have been drifting towards a CMS core that is only a core, much like Lucene is just a core search engine without being tied to a particular UI or data store. Today I might have stumbled across such a CMS core - riotfamily.org. I'll definitely look into it in more detail.
The downside of using a core is, of course, that you have to build supporting infrastructure around it - which is no big deal for someone like me who writes custom applications for a living :)
The CMS Matrix has a pretty extensive comparison of CMS products.