Kaitai Struct!

HxD is a carefully designed and fast hex editor which, additionally to raw disk editing and modifying of main memory (RAM), handles files of any size.

WinHex is a hexadecimal editor capable of opening disks, sectors, files (native support for FAT, NTFS, Ext2/3, ReiserFS, Reiser4, UFS, CDFS, UDF) and physical memory (RAM). It can read, write and extract data for a further forensic analysis.

010 Editor is a professional text/hex editor designed to edit any file, drive, or process on your machine. Using Binary Templates technology, 010 Editor allows a binary file to be parsed into a data structure that can be understood. Features an editor with standard Cut, Copy, and Paste commands, support for huge files, and unlimited Undo and Redo for all editing operations. Use the Inspector to easily edit data in a number of different formats.

FileAlyzer is a tool to analyze files, allows a basic analysis of files (showing file properties and file contents in hex dump form) and is able to interpret common file contents like resources structures (like text, graphics, HTML, media and PE).

A fast and clever open source HEX editor for Mac OS X.

HexEdit is a hexadecimal file editor for Microsoft Windows. HexEdit allows the user to view and edit any type of file, no matter what format it is saved in.

You got a binary file and don't know its contents. Or some software creates binary files where you have a specification but don't want to decode them manually.

Preon aims to provide a framework for dealing with binary encoded data; or more specifically, it aims to deal with situations in which bytes are considered to be overkill. The project is named after the "point-like" particles, conceived to be subcomponents of quarks and leptons. Let's just say very small particles that you don't see with the naked eye, but you probably rely on them without knowing.

The professional Hex Editor for Windows and Linux.