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There are a few binary Emacs packages for OS X floating around out there, but I always build it myself from the sources. This usually results in an Emacs that works the way I expect, rather than the way some "helpful" distributor thinks it ought to work.

I'll assume you have the developer tools and CVS installed, and know how to open Terminal and type some commands. Here are the commands you need to type:

cvs -z3 -d:pserver:anonymous@cvs.savannah.gnu.org:/sources/emacs co emacs

cd emacs

./configure --with-ns

make install

When this is complete, you'll end up with Emacs.app in the nextstep subdirectory of the emacs directory. You can run Emacs.app from there, or copy it to your Applications directory.