ghex (hex editor with GTK+3 GUI)

GHex allows the user to load data from any file, view and edit it in
either hex or ASCII. Useful for debugging problems with object code
or encodings. Also used by kids who cheat at computer games, by adding
score or lives to saved games.

Optional dependency: yelp. Required for viewing the built-in help
(Help->Contents, or F1 key). Alternatively, an HTML version of the
help is included in /usr/doc/ghex-$VERSION/html/index.html for your
perusal (in English only, though).

Note: Trying to copy/paste arbitrary binary data into or out of
ghex isn't going to work very well, because modern GTK+ insists the
copy buffer has to be UTF-8 encoded Unicode. The result is lots
of "\Uffffffff" (meaning "Invalid UTF-8") rather than the binary
data. There's nothing we can do to fix this without major surgery.