I went on a similar hunt a couple of years ago. I tried the one in XCode, didn't like it, used Delta Walker for a while but found it fiddly to set up and work with, and then found VisualDiffer. It works flawlessly and does what you want. You can find it in the App Store.
Richard E. Cooke wrote:
This is the (most excellent) software I use on Windows:
The are working on an OS X version - they already have Linux/Windows versions.
I'm actually waiting for their version to come out. I know their software is good on Windows, so I would rather wait for their MAC version than risk "trying out" something new.
That looks promising. I'll try the demo of Beyond Compare in Windows, which I have running as a virtual machine under Parallels Desktop. (Of course I still want an OS X version of such a utility, since firing up Parallels and then Windows is an utterly ridiculous way to approach the problem.)