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 Apple’s standard Human Interface Guidelines (HIG) than standard versions of the editor do.
See the Aquamacs project page for more information.