GNU Emacs is an editor well-known for its flexibility and high customization level; with Aquamacs, Mac users can benefit from all the features they have come to enjoy over the years from Emacs under a new, improved interface. All this is available as an open-source product which should make it even more appealing.
The application includes all that developers and programmers have come to expect from modern day editors: syntax highlighting for all the major programming languages like XML and C, code folding, built-in tabbed system, support for HTML and LaTeX, auto-complete (Code Sense), a choice of themes, export capabilities to HTML and PDF and much more.
Along with the standard Emacs set of features, Aquamacs adds spell checking for all opened documents, integration with clipboard operations, the freedom to define your own fonts and colors, as well as Unicode support.
Briefly, Aquamacs remains fully extensible, easily accessible for all its functions (thanks to the available shortcut keys) and an all-in-one programmer's / developer's editor.