An extensively-customized spin of Emacs for use on Apple's OSX.

From the EmacsWiki page: Aquamacs... is an Aqua-native build of the powerful Emacs text editor. By "Aqua-native," we mean more than just the fact that this version of Emacs runs as a standard OS X application. Aquamacs features extensive customization that enables it to conform better with Appleā€™s standard Human Interface Guidelines (HIG) than standard versions of the editor do.

See the Aquamacs project page for more information.