こんにちは @wasabeef_jp です
最低限、これだけ知っていればAndroidエンジニアとして語れるものを一覧にしています。

随時、このRepositoryを更新していきますので、参考にして下さい。
参照：wasabeef/ListOfAndroidLibraries

UI系のライブラリは含めてません

Networking

Name | Repository | License
--- | --- | --- | ---
Android Asynchronous Http Client | https://github.com/loopj/android-async-http | Apache License V2
Async Http Client | https://github.com/AsyncHttpClient/async-http-client | Apache License V2
ion | https://github.com/koush/ion | Apache License V2
OkHttp | https://github.com/square/okhttp | Apache License V2
Retrofit | https://github.com/square/retrofit | Apache License V2
RxNetty | https://github.com/ReactiveX/RxNetty | Apache License V2
Volley | https://android.googlesource.com/platform/frameworks/volley | Apache License V2

Image Loader

Name | Repository | License
--- | --- | --- | ---
Glide | https://github.com/bumptech/glide | License
ion | https://github.com/koush/ion | Apache License V2
Picasso | https://github.com/square/picasso | Apache License V2
Universal Image Loader | https://github.com/nostra13/Android-Universal-Image-Loader | Apache License V2
Volley | https://android.googlesource.com/platform/frameworks/volley | Apache License V2

Dependency Injections

Name | Repository | License
--- | --- | --- | ---
AndroidAnnotations | https://github.com/excilys/androidannotations | Apache License V2
Butter Knife | https://github.com/JakeWharton/butterknife | Apache License V2
Dagger(Square) | https://github.com/square/dagger | Apache License V2
Dagger(Google) | https://github.com/google/dagger | Apache License V2
RoboGuice | https://github.com/roboguice/roboguice | Apache License V2

FRP

Name | Repository | License
--- | --- | --- | ---
RxAndroid | https://github.com/ReactiveX/RxAndroid | Apache License V2
RxJava | https://github.com/ReactiveX/RxJava | Apache License V2

JSON

Name | Repository | License
--- | --- | --- | ---
Gson | http://google-gson.googlecode.com/svn/trunk/ | Apache License V2
ig-json-parser | https://github.com/Instagram/ig-json-parser | BSD
Jackson | https://github.com/FasterXML/jackson | Apache License V2

O/R Mapping

Name | Repository | License
--- | --- | --- | ---
ActiveAndroid | https://github.com/pardom/ActiveAndroid | Apache License V2
DBFlow | https://github.com/Raizlabs/DBFlow | MIT
greenDAO | https://github.com/greenrobot/greenDAO | Apache License V2
Sugar ORM | https://github.com/satyan/sugar | Satya Narayan
Shillelagh | https://github.com/pieces029/shillelagh | Apache License V2
Ollie | https://github.com/pardom/Ollie | Apache License V2

NoSQL

Name | Repository | License
--- | --- | --- | ---
Couchbase-Lite-Android | https://github.com/couchbase/couchbase-lite-android | Apache License V2
Realm | https://github.com/realm/realm-java | Apache License V2
SimpleNoSQL | https://github.com/Jearil/SimpleNoSQL | Apache License V2
SnappyDB | https://github.com/nhachicha/SnappyDB | Apache License V2

Pub/Sub

Name | Repository | License
--- | --- | --- | ---
EventBus | https://github.com/greenrobot/EventBus | Apache License V2
Otto | https://github.com/square/otto | Apache License V2

Logger

Name | Repository | License
--- | --- | --- | ---
Hugo | https://github.com/JakeWharton/hugo | Apache License V2
Timber | https://github.com/JakeWharton/timber | Apache License V2

Background Processing

Name | Repository | License
--- | --- | --- | ---
Android Priority Job Queue | https://github.com/path/android-priority-jobqueue | MIT
Tape | https://github.com/square/tape | Apache License V2

Fonts

Name | Repository | License
--- | --- | --- | ---
Calligraphy | https://github.com/chrisjenx/Calligraphy | Apache License V2

Video

Name | Repository | License
--- | --- | --- | ---
ijkplayer | https://github.com/bbcallen/ijkplayer | Apache License V2
mp4parser | https://github.com/sannies/mp4parser | Apache License V2

Parcelables

Name | Repository | License
--- | --- | --- | ---
AutoParcel | https://github.com/frankiesardo/auto-parcel | Apache License V2
Icepick | https://github.com/frankiesardo/icepick | Apache License V2
Parceler | https://github.com/johncarl81/parceler | Apache License V2

Functional Programming

Name | Repository | License
--- | --- | --- | ---
Retrolambda | https://github.com/orfjackal/retrolambda | Apache License V2

Social Networks

Name | Repository | License
--- | --- | --- | ---
ASNE | https://github.com/gorbin/ASNE | MIT
Facebook SDK | https://github.com/facebook/facebook-android-sdk | Apache License V2
Twitter Fabric | Unknown | Unknown

Showcases

Name | Repository | License
--- | --- | --- | ---
android-best-practices | https://github.com/futurice/android-best-practices | CC BY 4.0
Google/iosched | https://github.com/google/iosched | Apache License V2
JakeWharton/u2020 | https://github.com/JakeWharton/u2020 | Apache License V2
JamsMusicPlayer | https://github.com/psaravan/JamsMusicPlayer | Apache License V2
Talon-for-Twitter | https://github.com/klinker24/Talon-for-Twitter | Apache License V2