I had to compare 2 hefty directories, and was looking for a slick Diff tool to help me out. I really was looking for something with a slick GUI to help me sort through my potential mess of code and files.
After a bit of searching I realized that I had FileMerge already on my Mac as part of Xcode.
Once I fired up FileMerge all I had to do was drab the 2 directories I wanted to compare into each side of the GUI, then run a comparison. It was exactly what I needed for this task and saved me who knows how much time.