Perhaps I should be more thoughtful and give a reason for my answer. As with vi, emacs will be likely be installed on any non-windows system you encounter. I would hope a developer would also have it on windows. Works from the terminal. Works from X11. Carbon and Cocoa versions available. Syntax highlighting. Automatic code formatting. Split buffer editing. Support for shell processes within editor. Great hooks for debuggers, compilers, version control systems, etc. Great support for virtually every language ever created. You could read your email or news or whatever from it. Extremely, no, ridiculously customizable.
A side benefit is once you have gotten used to the somewhat odd keystroke controls for emacs, you will be pleasantly surprised when you discover many of them work in all mac applications.