That’s the compiler that comes with Apple’s XCode tools package. They’ve hacked on it a little, but basically it’s just g++.
You can download XCode for free (well, mostly, you do have to sign up to become an ADC member, but that’s free too) here: http://developer.apple.com/technology/xcode.html
Edit 2013-01-25: This answer was correct in 2010. It needs an update.
While XCode tools still has a command-line C++ compiler, In recent versions of OS X (I think 10.7 and later) have switched to clang/llvm (mostly because Apple wants all the benefits of Open Source without having to contribute back and clang is BSD licensed). Secondly, I think all you have to do to install XCode is to download it from the App store. I’m pretty sure it’s free there.
So, in order to get g++ you’ll have to use something like homebrew (seemingly the current way to install Open Source software on the Mac (though homebrew has a lot of caveats surrounding installing gcc using it)), fink (basically Debian’s apt system for OS X/Darwin), or MacPorts (Basically, OpenBSDs ports system for OS X/Darwin) to get it.
Fink definitely has the right packages. On 2016-12-26, it had gcc 5 and gcc 6 packages.
I’m less familiar with how MacPorts works, though some initial cursory investigation indicates they have the relevant packages as well.