You can learn when you are free, and some projects are often available.
1. Tailoring the picture Cropimage-master
Http://stackoverflow.com/questions/18013406/is-com-android-camera-action-crop-not-available-for-android-jelly-bean-4-3
2. Imageloader asynchronous loading more fluent, can display a large number of pictures, when dragging the ListView does not appear card phenomenon
Https://github.com/novoda/ImageLoader
Https://github.com/nostra13/Android-Universal-Image-Loader
http://blog.csdn.net/wwj_748/article/details/10079311
3. Android-edittext-validator-master Verify input information
Https://github.com/vekexasia/android-edittext-validator
4. Titanium_appsflyer_android_module
For app Stats
http://www.appsflyer.com/
5. Httpmime HttpClient is a sub-project under Apachejakarta Common that can be used to provide efficient, up-to-date, feature-rich support for HTT client Programming Toolkit for P protocol
6. Gpuimage Filter Artifact
7. Google-gson-2.2.4-release JSON parsing
8. Datadroid-master to simplify data management in Android apps
9. Android-pulltorefresh-master drop-down refresh artifact
10. parse-starter-project-1.5.0
Photoprocessing-master A Demo of how can process photos leveraging the NDK
Circleimageview-master Custom ImageView
Ionicons-master.