I have to admit that, even though I have tried on several occasions to dive head first into APEX, so far I have only wet my feet. I have played a little with APEX 4.0, liked the development experience [a lot better at least than the 3.x world] and found that many concepts are the same, whether your do APEX, ADF or [presumably] .NET web application development.
One of the nice features in APEX 4.0, that allows for more more organized, better structured development with lots more reuse potential, is the plugin framework. Through this framework, interesting and reusable pieces of functionality can be shared within and across applications – as well as across the APEX developer’s community. And that community is something to be reckoned with with many, many developers around the world.
Some very enthusiastic and quite active members of the APEX community work at AMIS. And they have produced a number of plugins, and shared several on the Plugin Directory: http://www.apex-plugin.com/index.php.