fq is inspired by the well known jq tool and language and allows you to work with binary formats the same way you would using jq. In addition it can present data like a hex viewer, transform, slice and concatenate binary data. It also supports nested formats and has an interactive REPL with auto-completion.
It was originally designed to query, inspect and debug media codecs and containers like mp4, flac, mp3, jpeg. Since then it has been extended to support a variety of formats like executables, packet captures (including TCP reassembly) and serialization formats like JSON, YAML, XML, ASN1 BER, Avro, CBOR, protobuf.
In summary it aims to be jq, hexdump, dd and gdb for files combined into one.
A binary editor written in Go that tries to act like vim. Text based. Portable. Insert/replace/delete individual bytes. Implements internal search.
Convert any number into different numerical bases, such as binary, trinary, octal, and base-20.
A hex editor that uses the QT toolkit for editing very, very large files - files larger than the amount of RAM in your system, in fact. Very useful.
A cheat-sheet for reverse engineering malware, by Lenny Zeltser.
A disassembler for Windows. One of the best in the field, and v2.0 just went beta. It's shareware, though it doesn't appear to lack any functionality if you haven't purchased it yet.
A website that takes input in one form and converts it into others for your use and edification.
Upload a file to this site and it not only takes the file apart like a hex editor, but it also implements a number of different visualization techniques that translate the binary into visual pictures. The visual feedback is highly useful in the process of reverse engineering, because people's brains just weren't wired to read hex.
BinNavi is a tool for performing static and dynamic reverse engineering of executables for a number of platforms - x86, ARM, PowerPC, and MIPS. Can connect to remote debuggers on other systems to examine running code. Extensible with scripts, can annotate call flow graphs. The source is on Github: https://github.com/google/binnavi