JPEXS Free Flash Decompiler is open-source Flash SWF decompiler
...It also supports converting SWF content into FLA format for further editing. Its integrated interface provides resource previews, script editing, and multiple export options. Built with Java, it runs on Windows, macOS, and Linux and is useful for software preservation, reverse engineering, and Flash content recovery.
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.