What's a good Mac equivalent of WinMerge?
You can use FileMerge, Apple's diff solution. It's free and it comes with every Mac OS X install.
The only downside is that you have to install the Developer Tools. You can find them on your DVD install that came when you bought your Mac (Snow Leopard or earlier). You can also get the developer tools from the App Store if your version of the OS supports that.

Then, you can find it at /Developer/Applications/Utilities/FileMerge.app