Aquamacs vs Carbon Emacs
Aquamacs and Carbon Emacs are Emacs clones for the Mac OS X designed to have a native look and feel. Both Aquamacs and Carbon Emacs are based on Emacs code and can be described as builds or distributions of Emacs rather than forks: "Aquamacs uses the same code-base as GNU Emacs. As GNU Emacs evolves, so does Aquamacs. We keep our code-base synced. Actually, building Aquamacs involves compiling Emacs from the GNU Emacs CVS. Aquamacs contributes back to the GNU Emacs project." 
Aquamacs and Carbon Emacs have more in common than not since both are essentially a GUI wrapping around Emacs. The differences in features, therefore, are mostly superficial:
 Look and Feel
Aquamacs "will feel and behave mostly like an Aqua program" ; Carbon Emacs behaves like a Carbon application.
 Full Screen Mode
Both Aquamacs and Carbon Emacs support full-screen mode.
 OS X Shortcut Keys
Both Aquamacs and Carbon Emacs support OS X shortcut keys. In Carbon Emacs, these shortcut keys must be enabled by setting the
 Tab Support
Aquamacs has tab support. Carbon Emacs does not.
 Multilingual Support
Aquamacs supports OS X's multilingual input out of the box. In Carbon Emacs, multilingual input can be enabled by setting
Aquamacs has had over 200,000 downloads. 
"Aquamacs Emacs has been developed by David Reitter (maintainer, email@example.com). GNU Emacs is the work of Richard Stallman and many other developers, including Andrew Choi who first ported GNU Emacs to the Mac. Kevin Walzer co-founded the project and wrote the initial documentation. Nathaniel Cunningham contributed code for the tabs. Adrian Chromenko contributed toolbar artwork." 
Carbon Emacs is maintained by Seiji Zenitani.