APK Studio
Open-source, cross platform Qt based IDE for reverse-engineering
Open-source, cross-platform Qt based IDE for reverse-engineering Android application packages. It features a friendly IDE-like layout including code editor with syntax highlighting support for *.smali code files. If you are just willing to quickly decompile an Android app, you can make use of DeAPK - Online APK Decompiler which lets you decompile an Android app using apktool and jadx.