Many developers moving from Windows to OS X will find that one valued application "Beyond Compare" has no mac equivalent.
Recently a new visual diff application has come out that works very well (for a 1.0). It integrates nicely with Textmate and the OS X terminal. While not as feature-complete and polished as Beyond Compare it does give you the meat & potatoes of what you need - folder and file diffing.
I encourage you to try it out but remember this is 1.0 software. For example, see my screenshot of a folder diff. Notice how the file/folder names are abbreviated even when there is ample room for display.
