There are 2 repositories under flipnote-studio topic.
The Cross-Platform Flipnote Encoder and Signer written in .NET! Available for Windows, Linux and OsX
Some people may know it as FlipnoteDesktop, a fan-made PC version of Flipnote Studio.
Flipnote Studio Player for Nintendo DS
Source code of Flipnote Encoder
Tool for debugging audio from Flipnote Studio 3D's DSi Library notes
Flipnote ID is a program written in Python 3 that can get the names, Studio IDs, and more from the metadata of Flipnotes
A tool for modifying Flipnote Studio ID (FSID)
A Go library to parse Flipnote Studio animation files
A simple tool to create commands for Hatenatools (by Pbsds) on the fly (this project is old and outdated, ignore the poor code quality)
Flipnote Studio recreated in Python 3
The best FLStudio Activation application on the whole github!