I'm working on using QT's plugin system to create libraries for my software. Here is the link to the doc section that talks about QT Plugins:

http://doc.trolltech.com/4.3/plugins-howto.html

Here is a post I made earlier with sample code for a plugin:
http://www.qtcentre.org/forum/f-qt-p...ghlight=plugin

*Note: You will need to fix the createWidget declaration in interface.h by making it a pure virtual function, like wysota was saying .

Honestly, after reading through the plugin page, going over the paint example in the docs and creating a sample, it is really easy to extend your app through the QT Plugins.