Format Description Tools (File Formats): open-source projects that exist because someone reverse engineered a closed product, sorted by GitHub stars.
Hex editor for reverse engineers with a pattern language for binary formats
jq-like tool, language and decoders for inspecting binary formats
Declarative language to describe binary formats and generate parsers
Pattern files describing many binary file formats for ImHex
Library of .ksy descriptions of hundreds of binary file formats
Compiler that turns Kaitai Struct .ksy format descriptions into parsers for many languages
Extracts and decodes hundreds of legacy file, archive and image formats
Assembler/disassembler for Windows keyboard layout DLLs
Reverse-engineered Serum 2 synth preset format; converts to/from JSON
General approach for reverse-engineering Xilinx FPGA bitstream formats
Nextion .zi font format documentation, parser, generator and editor
XNA xnb/xact file parser and FEZ extractor/viewer
Format documentation and scripts from the libyal project
Documentation for Flipnote Studio 3D formats
Tools and specs for reverse-engineered game file formats
Python 3 library to read and write XBE, the original Xbox executable format
Reverse-engineered documentation of the Falcon sequence (FSEQ) v2 file format
Byte-exact codec for Cisco Packet Tracer .pkt/.pka lab files
Reverse-engineered Gravitrax marble run course file format
Library for viewing, extracting and validating data inside original Xbox XBE executables
Decoder for the proprietary .rkd telemetry format of Race-Keeper video recorders
Notes and resources for reverse engineering Shockwave/Director files
Reverse-engineered RLF log format of REMUS underwater drones
Reads and writes Garmin's undocumented GARMIN.SET device settings file