The best free alternative to 010 Editor is ImHex, which is also Open Source. If that doesn't suit you, our users have ranked more than 25 alternatives to 010 Editor and many of them is free so hopefully you can find a suitable replacement. Other interesting free alternatives to 010 Editor are HxD, Malcat, wxHexEditor and Hex Fiend.
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
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.
Proprietary, does not contain a lot of the features of 010
Malcat is a feature-rich hexadecimal editor / disassembler / decompiler for reverse engineers and malware analysts. It has support for many file formats and CPUs, can scan for signatures, decrypt/decompress data, diff files and run python scripts.
Hex Editor. Support editing or comparing (diff) very large files up to 2^64 bytes. Cross-platform Linux, Mac OS, Windows.
Okteta is a simple editor for the raw data of files. This type of program is also called hex editor or binary editor. The data is displayed in the traditional view with two columns: one with the numeric values and one with the assigned characters.
Bless is a high quality, full featured hex editor. It is written in mono/Gtk# and its primary platform is GNU/Linux. However it should be able to run without problems on every platform that mono and Gtk# run on.
Lacks most of the additional expert-ish features of 010, however it's otherwise a decent hex editor
BinEd is a hex editor tool, which allows to directly browse and edit content of any file or binary data.
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.
Kaitai Struct is a formal language for binary format specification that can be compiled into parser code in Java, JavaScript, Python and Ruby.
Free / open source replacement, does anything that 010 Editor does (format parsing, etc), and lots more (parser generation)
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High-quality open-source cross-platform editor with all of the features I've seen in 010. It also contains a large amount of additional features ontop of 010's.