Retrofit is an wonderful network library to deal with api's and Rx java can be used to do async task operations effectively. Using retrofit with RxJava will results in seamless network calls in mainthread. Retrofit can be used without RxJava, though to perform huge api calling and to do api calls in background, they pair a better combination.
CustomFontLib is an Android Library to help adding custom fonts to Android Views.
WaspDB is a pure Java key/value (NoSQL) database library for Android. It supports AES256 encryption for all the disk storage.