More than 5.500 extension exist currently for TYPO3 CMS (12/2012). With them TYPO3 CMS can be extended in any order.
Currently I know a lot of extensions just like that, and so I can fit them to existing conditions and wishes of my clients. Is something not configurable by TypoScript or Flexforms, I can work directly in the code.
Since 2008 I'm also writing own extensions. That makes sense, especially if the wishes of my clients are too individual or a fitment of existing extensions would takes longer than a new development.
During extension development my quality claim is high:
I present several of my extensions in the area References.