I've recently had a need to do a bit of lisp editing and I found the nifty Ready Lisp package for OS X, which is great, except Aquamacs automatically uses a proportional font (which is idiotic, IMHO) and I want to change it to a monospace font. However, I'm not really much of an EMACS user, and the preferences menu in Aquamacs is less than clear on where and how one might make such a change.
This is what I have in my .emacs for OS X:
(set-default-font "-apple-bitstream vera sans mono-medium-r-normal--0-0-0-0-m-0-mac-roman")
Now, I'm not sure Bitstream Vera comes standard on OS X, so you may have to either download it or choose a different font. You can search the X font names by running (x-list-fonts "searchterm") in an ELisp buffer (e.g. *scratch* - to run it, type it in and then type C-j on the same line).