While BBEdit has a great tool for comparing and applying changes between files, it does not generate standard .patch files that can be used for bug reports, code fixes, and change logging.
BBDiff will compare the contents of the frontmost window to the contents of the window just behind it, and will use the diff command line tool to generate diff output which is then pasted into a new BBEdit window for saving or pasting into a Web site.
The command line called by BBDiff by default is:
diff -up newfile oldfile
You can modify this by simply changing the diffopts property at the top of the script.