These are collections of links to sites that we have found useful and informative.
www.dzone.com - A good reference site for developers. The Free Reference Cards are very useful, especially for new developers.
download.oracle.com/javase/tutorial/ - The official Java tutorials, now provided by Oracle.
developers.android.com - Everything you need to know to start developing on Android.
code.google.com/appengine/docs/ - Docs for developing on Google's cloud environment.
SDKs - Software Development Kits
IDEs - Integrated development environments
Netbeans - http://netbeans.org/downloads/
Eclipse - http://www.eclipse.org/downloads/
Ant - http://ant.apache.org/
Maven - http://maven.apache.org/
Hudson - http://hudson-ci.org/
Coding Quality and Testing Tools
Cobetura - http://cobertura.sourceforge.net/
JUnit - http://www.junit.org/
Sonar - http://www.sonarsource.org/