Hexedit is a very powerful tool that shows a file both in ASCII and in hexadecimal. On Linux system, vi/vim can only modify ASCII files, however, Hexedit can modify not only ASCII files but also binary files.
The link http://rigaux.org/hexedit.html shows how to use it.
On CentOS 5.5 system, I cannot use yum to install it(but it can be installed by yum on Fedora 15). The below are installation steps.
# wget http://rigaux.org/hexedit-1.2.12.src.tgz
# gunzip hexedit-1.2.12.src.tgz
# tar xvf hexedit-1.2.12.src.tar
# cd hexedit
# make install