Working with sensors:
List of available sensors on Android
Showing dialogs :
How To display Alert Dialog in Android
How To display a simple Toast Notificatin in Android
How To display a Custom Toast Notification in Android
Working with images from web:
How to load an image from Web on Android
How to use AsyncTask to load images on Android
Working with lists:
How to create a listview with custom adapter on Android
Other:
Best books for Android development
Android market alternatives