HexEdit is a Macintosh hexadecimal file editor that handles both data and resource forks. It is the most widley used editor of itis kind on the platform, and can handle file sizes of many Gigabytes with ease, yet has memory requirements of less than a Megabyte (with color turned off). It can compare whole files for similarities or differences and will soon be able to do so for any subset within the files.
HexEdit is Open Source and is available for free. You can find more information at the SourceForge Web site.