Tweak is a hex editor.  It allows you to edit a file at very low level,
letting you see the full and exact binary contents of the file. It can be
useful for modifying binary files such as executables, editing disk or
CD images, debugging programs that generate binary file formats incorrectly,
and many other things.

WWW: http://www.chiark.greenend.org.uk/~sgtatham/tweak/