Architecture:Ignition – helps you getting your Android applications off the ground quickly, by offering ready-to-use components and utility classes.
GreenDroid – It is intended to make UI developments easier and consistent through your applications.
Spring For Android – an extension of the Spring Framework that aims to simplify the development of native Android applications.
DroidParts – a framework that includes DI, ORM, Logging, Rest Client and more.
ORM:OrmLite – provides some lightweight functionality for persisting Java objects to SQL databases
Active Record for Android - lightweight persistence library for Android.
GreenDao – open source project to help Android developers working with data stored in SQLite.
JEPLDroid (port of JEPLayer to Android)
Design:Android Patterns – a set of interaction patterns that can help you design Android apps.
Google Android Design Guide – official android design guide by google.
Android App Patterns – 1225 app screenshots in 29 categories all in one.
Android 4.0 Design Startkit – some startkit Photoshop PSD’s.
Android Niceties – a collection of screenshots encompassing some of the most beautiful looking Android apps.
Android Holo Colors Generator – allows you to easily create Android components with your own colors for your Android application.
ICS Apps – icecream sandwich app screenshots.
Dependency Injection:RoboGuice – smoothes out some of the wrinkles in your Android development and slims down your application code.
Android Annotations – it takes care of the plumbing, and lets you concentrate on what’s really important. By simplifying your code, it facilitates its maintenance.
Networking:Loopj – an asynchronous callback-based Http client for Android.
Volley – it manages the processing and caching of network requests and it saves developers valuable time from writing the same network call/cache code again and again.
UI:ActionBarSherlock – designed to facilitate the use of the action bar design pattern across all versions of Android with a single API.
NineOldAndroids – library for using the Honeycomb (Android 3.0) animation API on all versions of the platform back to 1.0.
Pull To Refresh – this project aims to provide a reusable Pull to Refresh widget for Android.
Android Validator – form validator library.
Progress Wheel – a custom component for Android intended for use instead of a progress bar.
Scroll Bar Panel – allows to attach a View to a scroll indicator.
View Page Indicator – widgets compatible with the
ViewPagerfrom the Android Support Library and ActionBarSherlock.
View Badger – a simple way to “badge” any given Android view at runtime.
Universal Image Loader – powerful and flexible instrument for asynchronous loading, caching and displaying images.
ColorPicker – to create color picker in preferences.
Segmented Radio Button – implementation of iPhone’s segmented control.
PhotoView – implementation of ImageView that supports zooming, by various touch gestures.
Smart Image View – ImageView replacement which allows image loading from URLs or contact address book, with caching.
Android Lock Pattern – users can define their own lock pattern ‒ which is a combination of 4+ dots.
Testing:Android Mock – a framework for mocking interfaces and classes on the Dalvik VM.
Robotium – test automation framework that has full support for native and hybrid applications.
Tools:Application Crash Report – ACRA – a library enabling Android Application to automatically post their crash reports to a GoogleDoc form.
Android Studio – a new Android development environment based on IntelliJ IDEA.
Some more helpful:App Dev Wiki
Open Intents Android Libraries
Vogella Useful Android Libs
This List will be updated soon Stay tuned.