Every since I started using source control one of my favorite utilities on Windows has been Araxis Merge. It is certainly one of the better diff/merging applications I have used, plus it does folder comparisons and synchronization which is very handy when trying to sync up files on different servers. Since switching to the Mac I've been looking for something comparable and today I was happy to find out that my search may soon come to an end. I've just downloaded the early-access test release of Araxis Merge for Mac OS X. I haven't really put it through its paces yet, and according to the download page it is still a little rough around the edges, but so far it looks pretty sweet. If it ends up being anything like the Windows version I'll be one happy camper.
If your looking for an awesome diff/merge application for Windows or Mac definitely check out Merge. It isn't free, but I've found it well worth the money. (You can download a Windows trial from the website. If your looking for the early-access test release for the Mac you best bet is to contact Araxis via their website.)