Changing the font in Aquamacs?

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).

In Aquamacs 2.1, you can set the font through Options->Appearance->Font for Text Mode... That brings up the standard font chooser window, choose the font you like. Then, when you exit out of emacs (C-x C-c) you'll be prompted to save options, hit "y".