MacTeX is an open source TeX distribution for Mac OS X.
MacTeX is a redistribution of TeX Live. While TeX Live is designed to be cross-platform (running on Unix, Mac OS X, and Windows), MacTeX includes Mac-specific utilities and front-ends. It is also pre-configured to work out-of-the-box with Mac OS X as it provides sensible defaults for configuration options that, in TeX Live, are left up to the user to allow for its cross-platform compatibility.
For more information see: MacTeX.