The Adobe Consulting Services Team has generated a series of AEM code samples. These samples are well commented pieces of code to help AEM Developers understand the key building blocks of AEM. The samples are available here: http://adobe-consulting-services.github.io/acs-aem-samples/.
The Adobe AEM Community team creates Developer How To articles. You can find the articles here.
A Maven template to create a new AEM project as starting point to develop your own functionality is available here.
Touch UI Optimized code samples are available here.
Additional code samples are available: