File comparison tools are pretty critical to what I do. Most of the time, I rely on command-line diff (under Mac Terminal, or Windows Cygwin shell). But there are times when only a graphical comparison tool will do; such as recently, when I changed a vast amount of code, only to have introduced a new bug that was proving very difficult to track down. In these cases, you need to compare old and new code, one file (or folder) at a time, painstakingly looking at each change until you figure-out what went wrong.
A friend introduced me to Beyond Compare for Windows - which I liked enough to buy a license for. Great software!
However, now that I've changed my focus to do as much of my core development as possible on the Mac (avoiding Parallels where possible as it really is very slow compared to native Mac tools), I've been hunting around for a good graphical diff for Mac. And I have now found that tool - it is called Changes. Give it a try - tremendously good value, I'm very pleased to have bought a license!
Incidentally, I'm quite amazed at how much faster the shell on Mac works compared to Cygwin on Windows. I use a lot of scripting as part of my software development setup; one of the reasons to moving over to doing as much work as possible on the Mac is that the faster scripting makes a big difference to my productivity. I wonder why Cygwin is so much slower than the Mac Terminal window?