If you have done web development on a PC in Dreamweaver you have probably used WinMerge to check for differences between local and remote files. Since I moved to the Mac a few years ago I have been neglectful in finding a similar tool for the Mac. Thanks to some changes in my current day job I now have a larger team and more of a need for getting my act together on this.
The tool I have decided on in a free one called DiffMerge. Part 1 of the install is the typical Mac install (drag a file to the Applications folder). Part 2 is a little more detailed (but dead simple). There is a readme.txt file in the installer image with instructions for copying 2 files to facilitate command line usage (You will need this for Dreamweaver).
Once you have done the installation and followed the readme.txt fire up Dreamweaver and click Dreamweaver > Preferences > File compare and set the path to your equivalent of the screenshot below: