I\'ve recently begun building version 2 of my year-old custom MVC framework. It\'s worked great for me on a number of projects, but I definitely see room for improvement. It
Personally Zend FW is my favorite. It kicks ass. Period. That said though, you have to be a coder at heart to be able to appreciate it. It can get quite abstract.
To answer your question; Zend FW is very extendable, everything - really everything - can be extended. Class naming convention is; class Zend_This_IsAn_Example is found in /library/Zend/This/IsAn/Example.php which you would extend as Whatever_This_IsAn_Example which you would put in /library/Whatever/This/IsAn/Example.php It also has autoloading that you can enable, so you won't have to require_once classfiles anymore.
mvc structure;
/application /application/controllers /application/etc /application/models /application/views /library /library/Zend /library/Custom /public
Check out http://framework.zend.com/manual/en/introduction.html and http://framework.zend.com/docs/quickstart