Hed is a free hexadecimal editor for POSIX systems with vi-like controls and designed to efficiently handle infinitely large files in conjunction with operations like inserting in the middle of the file.
Hed was originally written by Petr Baudis <pasky@ucw.cz>, now maintained by Petr Tesarik <petr@tesarici.cz> and distributed as free software under GPLv2.
The latest release is hed-0.4 (2009-02-03). It's pretty old, so better get the current version from git.
You can also grab a tarball of the latest development version. See above for the details.
Hed is currently beta-quality software. It should be well usable but not very well tested - and only you can help with that! Please do not use it on critical data without backups, though.
You can get details on how to grab the latest version at its gitweb page (use git clone URL). Click on the snapshot link to get a tarball with the snapshost of the current version.
You are encouraged to send patches to the maintainer. Also, you can freely push to the mob branch of the repository - see details.