These are collections of links to sites that I have found useful and informative.
Resources
www.dzone.com - A good reference site for developers. The Free Reference Cards are very useful, especially for new developers.
developers.android.com - Everything you need to know to start developing on Android.
SDKs - Software Development Kits
Java - https://www.oracle.com/java/technologies/
Android - http://developer.android.com/sdk/index.html
IDEs - Integrated development environments
Android Studio - developers.android.com
Netbeans - http://netbeans.org/downloads/
Eclipse - http://www.eclipse.org/downloads/
Devops Tools
Jenkins - https://www.jenkins.io/
Coding Quality and Testing Tools
Cobetura - http://cobertura.sourceforge.net/
JUnit - http://www.junit.org/
PMD - http://pmd.sourceforge.net/
Sonar - http://www.sonarsource.org/