So I don't what you are asking about advanced features beyond just viewing or editing. So I will answer about some commands you can use that are free and open source that are readily available in the linux environment, and you can probably find for other systems.
For quickly viewing a file in hex, you can use the hexdump command.
For comparing two files, you can use the diff command. You can run hexdump on the two files and save the output and run diff on the hex output to visually see where differences occur.
For editing and comparing at the same time, you can use the various vi editors in hex mode, split screen. vim has a diff mode as well, although I don't think I have run a vimdiff in hex mode for editing and comparing purposes simultaneously. I usually just run on the command line for comparisons and more rarely go into an editor to change some numbers.
I think you need to explain what you want to do if you want a more specific program.