hexyl is a simple hex viewer for the terminal. It uses a colored output to distinguish different categories of bytes (NULL bytes, printable ASCII characters, ASCII whitespace characters, other ASCII characters and non-ASCII).

DroiD64 is a graphical file manager for the contents of D64, D67, D71, D80, D81, D82, D88, T64 and LNX files. Examine your disk images in a fine-grained way to see what's in there.

A binary editor written in Go that tries to act like vim. Text based. Portable. Insert/replace/delete individual bytes. Implements internal search.

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.

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.