My favorite diff tool is FileMerge, one of Apple’s developer tools, which can be accessed from the command line as opendiff.

But from the command line, my (next most) favorite diff tool is colordiff. It’s called that because it color-codes the output when it’s used interactively, making it IMHO easier to see what’s changed. Actually, colordiff is just a wrapper around the real diff tool.

Another nice tool is dwdiff, which is compares two documents and highlights the different words rather than the different lines. So does wdiff.

(Back to my list of Unixy tools for the Mac.)