If you're looking for Mac-only software (which is a much shorter list than Mac-supported software then I only have a few things to recommend.
Generally, I try to use cross-platform tools on every platform, but these outliers on OS X are so good that I'll gladly use them over their cross-platform competitors:
I use Kaleidoscope to compare individual files, or file trees for differences so I can be more aware of what has changed between them. Just yesterday I was using this tool to compare new additions somebody made in version 1.2.1 of a file and to isolate those changes to see if I could try adding them to version 1.5.0, even though those two files have changed quite a bit.
There are many different image compression algorithms, and imageOptim uses all of them :D When you optimize an image using imageOptim it runs a tournament between all different algorithms and gives you the ultimate winner. Because all sorts of images compress differently, it's not always going to be the same algorithm that crunches all of them. This is easily the most important Mac-only utility, because there's no equivalent tool I've found on any other OS.
Coda strikes the perfect balance between editor and IDE, and it includes an FTP view, an Editor view (with a browser-preview), a Terminal view, and a MySQL view - meaning when I'm working on websites I can set Coda to fullscreen and:
All without leaving my editor :D Because I can work on all aspects of a website from the same app, setting it to fullscreen is like putting myself into 'Productivity Mode' and eliminating distractions.
There are things about every editor that annoy me, but for all the editors I've tried on all OSes, Coda gets the least things wrong and it's the comfiest editor I've ever used. I wish this was ported to Linux :D