GNU poke: The extensible editor for structured binary data

ImHex is a free, featureful Hex Editor with many tools often only found in paid products. It includes a custom built scripting language for highlighting and decoding binary data, a disassembler, extensive file analysis tools and much much more

Kaitai Struct is a formal language for binary format specification that can be compiled into parser code

pkgsrc is a framework for building over 17,000 open source software packages.