There are hex editing programs, including programs with that very name, available for most operating systems and computers. Some of these programs edit system memory in hexadecimal (hex), some edit files, and some do both. I just did a quick search in Google and found:
  • HexEdit for Unix at
  • HexEdit for Macintosh at
  • HexEdit for Windows at
...and there are many more.

Minimalists can use DOS DEBUG, while those with (somebody else's) money to blow can buy Symantec's Norton Utilities. A high-powered programmer's editor like Visual Slick Edit will also edit files in hex, as an option.